By which or in which can be confusing because both phrases are grammatically correct, but they serve different purposes. The correct choice depends on the meaning of your sentence and the relationship you want to describe.
By which usually refers to a method, process, means, or way through which something happens. For example, “This is the method by which the machine operates.” Here, by which means “by the method through which.”
On the other hand, in which generally refers to a place, situation, condition, period, or context where something happens. For example, “This is the room in which we held the meeting.” In this sentence, in which refers to the room where the meeting happened.
Understanding this difference can make your writing clearer and more natural. Many learners use these phrases interchangeably because both contain the relative pronoun which. However, the preposition by or in changes the meaning completely. Choosing the right phrase depends on whether you are talking about how something happens or where, when, or under what circumstances it happens.
This guide will explain by which vs in which, their meanings, grammatical uses, sentence patterns, and everyday examples. By the end, you’ll know exactly when to use each expression and avoid common mistakes.
Choosing between by which and in which can feel confusing at first. Both expressions appear in formal English, especially in relative clauses. They also often follow nouns such as method, process, system, situation, and place.However, by which and in which don’t communicate the same relationship. The correct choice depends on what you want the sentence to explain.
Use by which when you want to describe a method, means, process, or mechanism. It often answers the question “How?”Use in which when you want to describe a place, situation, condition, context, or period of time. It often answers “Where?” “When?” or “In what situation?”
That small difference matters. Choosing the wrong preposition can change the meaning of an otherwise grammatical-looking sentence.This guide explains by which or in which, including their meanings, differences, sentence patterns, common mistakes, and practical examples. By the end, you’ll have a simple way to choose the right expression confidently.
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By Which or In Which: Quick Answer
The simplest rule is this:
By which usually refers to a method, means, process, or mechanism.
In which usually refers to a place, situation, condition, context, or period.
Consider these examples:
By which:
The process by which plants produce food is called photosynthesis.
Here, by which explains the process or mechanism through which plants produce food.
In which:
The laboratory in which the experiment took place was carefully controlled.
Here, in which identifies the place where the experiment happened.
The phrases can therefore be distinguished by the relationship they establish with the noun before them.
| Phrase | Main idea | Common question | Example |
| By which | Method or means | How? | The method by which data is collected is reliable. |
| In which | Place or setting | Where? | The room in which data is collected is secure. |
| By which | Process or mechanism | By what process? | The process by which the material changes is complex. |
| In which | Situation or condition | In what situation? | The situation in which they worked was difficult. |
| In which | Time period | When? | The year in which she graduated was memorable. |
The key isn’t the noun alone. Context determines which preposition makes sense.
What Does “By Which” Mean?
By which is a prepositional relative construction. It combines the preposition by with the relative pronoun which.
The phrase commonly describes the means or method used to accomplish something.
In simple terms, by which often means:
- by what method
- by what means
- through what method
- using which process
- through which mechanism
For example:
This is the method by which we calculate the total cost.
The phrase by which tells you how the calculation happens.
You can mentally expand the sentence:
This is the method by which we calculate the total cost.
becomes:
This is the method by what means we calculate the total cost.
That substitution helps reveal the relationship.
“By Which” Usually Answers “How?”
One useful test is to ask “How?”
Consider:
The system by which payments are processed has been updated.
Ask:
How are payments processed?
Answer:
By the system.
Therefore, by which fits the intended relationship.
Another example:
Researchers developed a technique by which they could identify the substance.
Ask:
How could they identify the substance?
Answer:
By using the technique.
Again, by which works naturally.
“By Which” With Methods
The word method frequently appears with by which because methods describe ways of doing things.
Examples include:
- The method by which the samples were tested was reliable.
- We need to understand the method by which the software detects errors.
- The method by which the company evaluates applicants has changed.
- Scientists studied the method by which the cells communicate.
In each example, by which introduces the means or procedure connected with the preceding noun.
“By Which” With Processes
By which also works naturally with processes.
For example:
The process by which water becomes vapor is known as evaporation.
The phrase explains the mechanism responsible for the change.
Another example:
The process by which the company approves applications takes several days.
The sentence focuses on how applications are approved.
This distinction becomes especially important in technical, academic, and professional writing.
“By Which” With Systems and Mechanisms
A system can describe either a method or an environment. Therefore, context matters.
Consider:
The system by which payments are processed is secure.
This sentence focuses on how payments are processed.
Compare it with:
The system in which customer records are stored is secure.
This sentence focuses on where or within what system records are stored.
The noun system appears in both sentences, but the relationship changes.
That’s why memorizing individual nouns isn’t enough. You need to understand the meaning of the entire sentence.
Examples of “By Which”
Here are several practical examples:
The formula by which the result is calculated is simple.
The phrase explains the calculation method.
The procedure by which applicants are selected is transparent.
The phrase describes the selection procedure.
The mechanism by which the device produces heat is fascinating.
The phrase explains how the device produces heat.
They created a system by which employees can submit requests online.
The system provides a means of submitting requests.
The process by which the material is recycled reduces waste.
The phrase describes how recycling occurs.
We examined the technique by which the artist created the effect.
The sentence focuses on the artistic technique.
In every case, by which connects a noun with the way something happens or gets done.
What Does “In Which” Mean?
In which combines the preposition in with the relative pronoun which.
It commonly refers to a place, situation, condition, context, activity, or period.
Depending on the sentence, in which can mean:
- in that place
- in that situation
- during that period
- within that condition
- within that context
- inside that setting
For example:
The office in which she works is downtown.
Here, in which refers to a physical place.
But it doesn’t always refer to a physical location.
Consider:
The situation in which they found themselves was unexpected.
Nobody is talking about a literal room or building. The phrase refers to a situation.
“In Which” Can Describe a Place
This is one of the easiest uses to understand.
The house in which I grew up has been renovated.
Here, in which means:
the house where I grew up
More examples include:
- The building in which they work is over 100 years old.
- The room in which we met was quiet.
- The laboratory in which the research was conducted is highly secure.
- The country in which she was born has changed significantly.
In these sentences, in which identifies the setting.
“In Which” Can Describe a Situation
The expression can also introduce a situation.
For example:
They faced a situation in which every option carried some risk.
Here, in which doesn’t describe physical space. It describes the circumstances surrounding the event.
Another example:
He found himself in a position in which he had to make a difficult decision.
The phrase identifies the circumstances or condition.
“In Which” Can Describe Time
In which can also refer to a period when something happened.
For example:
2025 was the year in which the company expanded internationally.
Here, in which means roughly:
the year when the company expanded internationally.
Other examples include:
- That was the period in which the industry changed rapidly.
- The decade in which the technology emerged transformed communication.
- She remembers the summer in which she learned to swim.
This use is common in formal writing.
By Which vs. In Which: The Main Difference
The main difference is the relationship expressed by the preposition.
By commonly expresses a means or method.
In commonly expresses location, circumstances, containment, or a period.
Therefore:
The method by which the problem was solved…
means the method used to solve the problem.
But:
The situation in which the problem occurred…
means the circumstances where or during which the problem occurred.
Here’s another comparison:
| Sentence | What it explains |
| The method by which the data was collected was accurate. | How the data was collected |
| The environment in which the data was collected was controlled. | Where or under what conditions it was collected |
| The process by which the material changes is complex. | How the material changes |
| The conditions in which the material changes are unusual. | Under what conditions it changes |
This is the heart of the by which or in which distinction.
When to Use “By Which”
Methods and Techniques
Use by which when the noun represents a method or technique.
Examples:
The technique by which the image was created is impressive.
The method by which the samples were analyzed produced consistent results.
The approach by which the team solved the issue saved time.
Each sentence explains how something was accomplished.
Processes and Procedures
Use by which when you describe a process that produces an outcome.
For example:
The procedure by which applications are reviewed takes two weeks.
The sentence doesn’t describe where applications are reviewed. It describes how the review occurs.
Another example:
The process by which metals are separated requires specialized equipment.
The phrase describes the mechanism or procedure.
Systems and Mechanisms
Use by which when you explain how a system functions.
The mechanism by which the lock operates is relatively simple.
The system by which orders are processed has been automated.
The process by which information reaches the server takes milliseconds.
The phrase connects the noun to its functional mechanism.
Means of Achieving Something
You can also use by which when something serves as a means of achieving a result.
They established a framework by which teams could share information.
The framework provides the means for sharing information.
Similarly:
The company introduced a policy by which employees could request flexible schedules.
The policy establishes the method or procedure through which employees make requests.
When to Use “In Which”
Physical Places
Use in which when the noun refers to a location or enclosed setting.
The room in which we met was bright.
The building in which the company operates is historic.
The laboratory in which the experiment occurred has restricted access.
In each case, the phrase identifies where something happens.
Situations and Circumstances
Use in which when describing circumstances.
She faced a situation in which she had to choose quickly.
They entered a period in which demand increased sharply.
He reached a point in which further discussion wasn’t useful.
These examples describe conditions rather than physical locations.
Time Periods
Use in which to connect an event with a specific period.
The year in which he graduated was unusually difficult.
The decade in which the technology developed changed the industry.
That was the month in which the project finally launched.
In everyday English, when may sound more natural:
That was the year when he graduated.
Still, in which remains grammatical and often sounds more formal.
Conditions and States
In which can also describe a condition or state.
The experiment was conducted under conditions in which contamination was unlikely.
Here, the phrase identifies the conditions surrounding the experiment.
Another example:
They created an environment in which employees could work independently.
The phrase describes the environment or circumstances.
“By Which” vs. “Through Which”
By which and through which can sometimes overlap, but they aren’t always identical.
By which often emphasizes a method or means.
Through which can emphasize a channel, route, medium, or process.
Compare these examples:
The method by which the results were calculated was accurate.
The channel through which the information was transmitted was secure.
The first sentence focuses on a method.
The second focuses on the route or medium used to transmit information.
Sometimes both phrases can work:
The process by which the information is transferred is automated.
The process through which the information is transferred is automated.
However, the choice can subtly change the emphasis.
When you mean “using this method,” by which is usually the clearer choice.
When you mean “via this channel or pathway,” through which may fit better.
“In Which” vs. “Where”
One of the most useful comparisons is in which versus where.
In many place-related sentences, where provides a more natural everyday alternative.
For example:
The house in which I grew up was small.
can become:
The house where I grew up was small.
Both sentences are acceptable.
The main difference is often style and formality.
In which tends to appear more frequently in formal, academic, legal, and technical writing.
Where is usually shorter and more conversational.
Compare:
| Formal style | More conversational style |
| The city in which she lives is growing rapidly. | The city where she lives is growing rapidly. |
| The room in which the meeting occurred was crowded. | The room where the meeting happened was crowded. |
| The country in which he was born has changed. | The country where he was born has changed. |
However, don’t automatically replace every instance of in which with where.
For abstract situations, in which may be more precise:
They faced circumstances in which immediate action was necessary.
Here, where may appear in informal writing, but in which clearly emphasizes the circumstances or conditions.
“By Which” vs. “That” and “Which”
English often allows different ways to express the same basic relationship.
Consider:
The method by which we calculate the value is reliable.
You could also write:
The method that we use to calculate the value is reliable.
The second version is often more natural in everyday English.
However, simply removing by doesn’t always preserve the original structure.
Compare:
The method by which the machine operates…
with:
The method which the machine operates…
The second sentence doesn’t work naturally because operate doesn’t take method as a direct object in that structure.
A better alternative would be:
The method that the machine uses…
This illustrates an important principle: relative-clause structures depend on the verb and the relationship between the words.
Why “By Which” Often Sounds Formal
You may notice by which more often in formal writing than casual conversation.
For example:
The procedure by which applications are evaluated is outlined below.
A casual writer might say:
The procedure used to evaluate applications is explained below.
Both communicate essentially the same idea.
The longer construction isn’t necessarily better. It simply creates a more formal grammatical structure.
You should choose the version that fits your audience.
Use by which when the formal relative clause adds precision or fits the style of the document.
Use a simpler construction when conversational clarity matters more.
Why “In Which” Often Sounds Formal
The same principle applies to in which.
For example:
The circumstances in which the decision was made remain unclear.
A more conversational version might be:
The circumstances surrounding the decision remain unclear.
Or:
The situation when the decision was made remains unclear.
The best choice depends on the sentence.
Formal writing often benefits from in which because it creates a precise relationship between the noun and the following clause.
However, don’t use it simply to make your writing sound sophisticated. Clear English beats unnecessarily complicated English.
Common Mistakes With By Which and In Which
Using “In Which” for a Method
One common error occurs when writers use in which after a noun that represents a method.
For example:
❌ The method in which the data was collected was unreliable.
If the intended meaning is the method used to collect the data, use:
✅ The method by which the data was collected was unreliable.
The problem is the relationship between method and the relative clause.
The sentence asks how the data was collected, so by which is appropriate.
Using “By Which” for a Place
The opposite mistake also occurs.
For example:
❌ The room by which we held the meeting was crowded.
If you mean the room where the meeting took place, use:
✅ The room in which we held the meeting was crowded.
The question is where the meeting happened, not how it happened.
Choosing Based Only on the Noun
A noun doesn’t always determine the correct phrase by itself.
Take system:
The system by which payments are processed…
Here, the system describes a method.
But:
The system in which the records are stored…
Here, the system represents the environment or framework containing the records.
The same noun can therefore work with different prepositions.
Context is more important than memorization.
Confusing “By Which” With “In Which”
Some sentences become confusing because writers choose a preposition without considering its meaning.
Ask yourself:
Am I explaining how something happens?
If yes, by which may be appropriate.
Am I explaining where, when, or under what circumstances something happens?
If yes, in which may be appropriate.
This simple distinction eliminates many errors.
Overusing Formal Relative Clauses
Another problem isn’t grammatical correctness. It’s unnecessary formality.
For example:
The manner by which the task was completed was impressive.
This is understandable, but:
The way the task was completed was impressive.
sounds more natural in everyday US English.
Similarly:
The location in which the event occurred…
can often become:
The location where the event occurred…
Formal constructions have their place. They shouldn’t appear in every sentence.
How to Choose Between By Which and In Which
You don’t need to memorize dozens of grammar rules. Follow a few practical steps.
Step One: Identify the Relationship
Look at the clause after the phrase.
Ask:
Does it explain how something happens?
Use by which.
Ask:
Does it explain where, when, or in what situation something happens?
Use in which.
Step Two: Try a Simple Substitution
Replace by which mentally with:
by what means
For example:
The process by which the device works…
Think:
The process by what means the device works…
The wording isn’t a natural replacement in the final sentence, but it helps you identify the meaning.
For in which, try:
in that place
or:
in that situation
For example:
The situation in which they found themselves…
Think:
the situation in that situation…
The grammar isn’t a direct substitution, but the relationship becomes clear.
Step Three: Look at the Noun
The noun before the relative clause provides an important clue.
Words such as these often work naturally with by which when the sentence describes their function:
- method
- process
- procedure
- technique
- mechanism
- approach
- means
- system
Words such as these often work naturally with in which:
- room
- building
- city
- country
- situation
- circumstance
- condition
- period
- year
- environment
But remember that these are patterns, not absolute rules.
Step Four: Read the Entire Sentence
Never choose the phrase based on one word alone.
Consider:
The process by which the company evaluates applications…
The process explains how evaluation happens.
Now consider:
The environment in which the company evaluates applications…
The environment explains where or under what conditions evaluation happens.
The surrounding words tell you what the sentence means.
By Which or In Which: Everyday Examples
Business Writing
The process by which invoices are approved has been updated.
This explains the approval procedure.
The office in which invoices are processed has moved.
This identifies the location.
Academic Writing
The mechanism by which the reaction occurs remains under investigation.
The sentence explains a scientific mechanism.
The conditions in which the reaction occurs affect the result.
The sentence explains environmental conditions.
Technology
The algorithm by which the application identifies errors has been improved.
The phrase describes the method used by the algorithm.
The environment in which the application runs requires additional memory.
The phrase describes the operating environment.
Everyday English
That’s the way by which the problem was solved.
This is grammatical in certain formal constructions, but everyday English would usually prefer:
That’s the way the problem was solved.
Another example:
This is the house in which I lived as a child.
A more conversational version is:
This is the house where I lived as a child.
Case Study: How One Preposition Changes the Meaning
Consider these two sentences:
The process by which the company operates is efficient.
The environment in which the company operates is efficient.
The first sentence focuses on the company’s method of operation.
The second focuses on the environment surrounding its operation.
Now change the noun:
The system by which employees submit requests is simple.
This means the method or system used for submitting requests is simple.
But:
The system in which employee records are stored is secure.
Now the system is treated as the framework or environment containing the records.
This demonstrates why by which and in which cannot be treated as interchangeable phrases.
The preposition establishes the relationship between the noun and the relative clause.
Can “By Which” and “In Which” Be Interchanged?
Generally, no.
They can only be exchanged when the resulting sentence still expresses the intended relationship, and in many cases changing the preposition changes the meaning or makes the sentence unnatural.
Compare:
The method by which the machine works…
and:
The method in which the machine works…
The second version doesn’t normally express the intended idea.
Now compare:
The room in which the machine operates…
with:
The room by which the machine operates…
The second sentence doesn’t naturally describe the room as the location.
The distinction comes from the basic meanings of the prepositions:
- by → means, method, agency, or route in certain contexts
- in → location, inclusion, circumstances, or time period
Therefore, replacing one with the other without considering meaning can create an incorrect or confusing sentence.
A Quick Decision Table
When you’re unsure, use this table:
| If you mean… | Consider… | Example |
| How something happens | by which | The process by which it works |
| The method used | by which | The method by which it was solved |
| The procedure used | by which | The procedure by which applications are reviewed |
| A physical place | in which | The room in which we met |
| A situation | in which | The situation in which they found themselves |
| A condition | in which | The conditions in which the test occurred |
| A time period | in which | The year in which she graduated |
| An environment | in which | The environment in which the program runs |
Common Sentence Patterns
Learning patterns can make these constructions easier to recognize.
Patterns With “By Which”
The method by which + subject + verb
The method by which the device operates is unusual.
The process by which + subject + verb
The process by which the material is produced is automated.
The system by which + subject + verb
The system by which payments are processed is secure.
The technique by which + subject + verb
The technique by which the image was created requires patience.
Patterns With “In Which”
The place in which + subject + verb
The building in which she works is historic.
The situation in which + subject + verb
The situation in which they found themselves was difficult.
The period in which + subject + verb
The period in which the study was conducted lasted six months.
The environment in which + subject + verb
The environment in which the software runs requires additional resources.
These patterns are useful because they reveal the underlying relationship rather than forcing you to memorize isolated examples.
Formal vs. Natural Alternatives
Sometimes the most grammatically formal option isn’t the most natural choice.
| Formal construction | Natural alternative |
| The method by which we solved it | The method we used to solve it |
| The process by which it operates | How it operates |
| The house in which I lived | The house where I lived |
| The year in which she graduated | The year when she graduated |
| The situation in which they worked | The situation they worked in |
This doesn’t mean the formal versions are wrong.
They may be preferable in academic, legal, technical, or professional contexts. The point is simply to choose language that matches your audience and purpose.
FAQS
Is “by which” grammatically correct?
Yes. By which is a standard grammatical construction. It commonly introduces a relative clause describing a method, means, process, or mechanism.
For example:
The method by which the results were calculated was reliable.
Is “in which” grammatically correct?
Yes. In which is also standard English. It commonly refers to a place, situation, condition, environment, or period.
For example:
The room in which the meeting took place was crowded.
What is the difference between “by which” and “in which”?
By which generally focuses on how something happens or is done.
In which generally focuses on where, when, or under what circumstances something happens.
A useful shortcut is:
How? → by which
Where, when, or in what situation? → in which
Which is correct: “the method by which” or “the method in which”?
When you mean the method used to do something, use the method by which.
Example:
The method by which the data was collected was effective.
The method in which usually doesn’t express that intended meaning.
Which is correct: “the situation by which” or “the situation in which”?
For describing the circumstances surrounding an event, the situation in which is normally correct.
Example:
They faced a situation in which they had to act quickly.
The phrase describes the circumstances, not the method.
Can “by which” mean “through which”?
Sometimes, yes. The two phrases can overlap when describing a means or process.
However, through which can place more emphasis on a channel, pathway, or medium.
For precise writing, choose the expression that best matches the relationship you want to describe.
Can “in which” be replaced with “where”?
Often, yes, especially when in which refers to a physical place.
For example:
The house in which I grew up…
can become:
The house where I grew up…
Where generally sounds more conversational, while in which can sound more formal.
Is “by which” formal?
Yes, by which often has a formal tone, especially in academic, legal, technical, and professional writing.
That doesn’t make it unnatural. It simply means simpler alternatives may sound better in casual conversation.
Is “in which” more formal than “where”?
Usually, yes.
For example:
The city in which he lives…
sounds more formal than:
The city where he lives…
Both are commonly acceptable when referring to a place.
Can “by which” describe a process?
Yes. In fact, this is one of its most common formal uses.
The process by which the material is recycled reduces waste.
The phrase explains how the process produces the result.
Can “in which” describe time?
Yes.
The year in which the company was founded was significant.
In less formal writing, when can often replace in which:
The year when the company was founded was significant.
Why can’t I simply use “which” instead?
The preposition carries meaning.
By which expresses a relationship involving by.
In which expresses a relationship involving in.
Removing the preposition can change the grammatical structure or make the sentence incomplete.
For example:
The method by which the problem was solved…
works because by connects the method with the means of solving the problem.
Does the noun always determine whether I should use “by which” or “in which”?
No.
The noun provides a clue, but the intended relationship determines the correct choice.
For example, system can work with both:
The system by which payments are processed…
The system in which records are stored…
The first describes a method. The second describes a framework or environment.
Conclusion
The difference between by which and in which becomes easier once you focus on the meaning of the sentence. Use by which when you are describing a method, means, process, or way something happens. Use in which when you are referring to a place, situation, condition, context, or period in which something occurs.
