Many English learners understand the words by, in, and which separately, but they often get confused when these words appear together as by which and in which.
The problem becomes even bigger when both phrases appear in formal writing, academic English, or longer sentences.
For example, compare these two sentences:
- This is the method by which we solve the problem.
- This is the room in which we had the meeting.
Both sentences contain which, but by and in create different meanings. The first sentence talks about a method or way. The second talks about a place.
This difference is important because these phrases are common in essays, reports, books, exams, business writing, and careful spoken English. They are also useful when you want to connect two ideas without repeating the same noun.
The good news is that the basic idea is quite simple. By which usually points to a method, process, means, rule, or sometimes a deadline or result.
In which usually points to a place, situation, condition, period, or setting. The preposition changes the relationship between the noun and the relative clause.
By the end of this guide, you will know when to use each phrase, how their sentence structures work, which mistakes to avoid, and how to choose the right one quickly.
Relative clauses are commonly used to add information about people, things, places, and situations without repeating information.
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What Does “By Which” Mean?
By which is made from the preposition by + the relative pronoun which.
It is generally used when by expresses a method, means, process, rule, or way of doing something. In simple English, you can often understand it as:
- by what method
- by what means
- according to which rule
- through which process
For example:
This is the method by which we calculate the answer.
Here, by which means “by this method” or “using this method.”
The phrase is especially common in formal writing. In everyday conversation, people often use a simpler form such as how, that, or a sentence ending in a preposition.
When should you use “by which”?
Use by which when the noun before it is connected to the idea of how something happens or is done.
Look at this example:
We need to understand the process by which the machine works.
The question behind the sentence is:
How does the machine work?
The answer is connected to a process, so by which fits naturally.
Grammar rule
The basic structure is:
noun + by which + subject + verb
Examples:
The method by which the teacher explained the lesson was simple.
The system by which payments are made is secure.
The noun before by which is called the antecedent. The word which refers back to that noun.
Here are several useful examples:
- This is the method by which we solve the problem.
- Scientists studied the process by which plants grow.
- The company changed the system by which customers place orders.
- That is the rule by which the game is played.
- We need a clear way by which students can submit their work.
- This is the road by which the trucks enter the factory.
- The machine uses a process by which water is cleaned.
- By which method did you reach this conclusion?
A common learner confusion
Students sometimes think by which means the same thing as in which because both can follow a noun.
They cannot always replace each other.
Compare:
The method by which the machine works is unusual.
This asks about the way or process.
But:
The room in which the machine works is very small.
This asks about the place.
The preposition changes the relationship completely.
Another important point is that by which can have other meanings. For example:
The year by which the project must be finished is 2027.
Here, by which means “before or no later than that time.”
So, do not think that by which always means “method.” Context matters.
What Does “In Which” Mean?
In which is made from the preposition in + the relative pronoun which.
It is commonly used when the noun before it refers to a place, situation, condition, period, environment, or context.
In simple English, in which often means:
- where
- inside which
- during which
- within which
- in that situation
For example:
This is the house in which I grew up.
Here, in which means:
the house where I grew up
The phrase is often more formal than where, which is why you may see it more often in academic or professional writing.
When should you use “in which”?
Use in which when the noun is something that naturally takes the preposition in.
For example:
the room in which we met
the situation in which he found himself
the year in which she graduated
The idea is that the action happens inside a place, within a situation, or during a period.
Grammar rule
The basic structure is:
noun + in which + subject + verb
Examples:
This is the city in which I was born.
We discussed the situation in which the mistake occurred.
Here are eight examples:
- This is the room in which we had our meeting.
- She returned to the house in which she was born.
- I remember the year in which we first met.
- He described the situation in which he had to make a difficult choice.
- That was the period in which the company grew quickly.
- We visited the village in which my grandfather lived.
- The case in which the documents were stored was locked.
- The environment in which children learn is very important.
“In which” and “where”
One of the easiest ways to understand in which is to compare it with where.
This is the school in which I studied.
This is the school where I studied.
Both sentences are correct, and they have almost the same meaning. Where is usually more natural in everyday English, while in which can sound more formal.
British Council explains that where can be used with places, while a phrase such as in which expresses the same relationship in a more explicit way.
A common learner confusion
Learners sometimes use in which simply because they see a place noun before it. But the real question is whether in makes sense.
For example:
The book in which I found the note was old.
This works because the note was found in the book.
But:
The book by which I found the note was old.
This would normally suggest a different relationship involving by, such as a method or means, so it does not express the intended meaning.
Difference Between “By Which” and “In Which”
The easiest way to understand the difference is to focus on the relationship between the noun and the action.
By often explains how, by what method, or according to what rule.
In often explains where, within what situation, or during what period.
| Phrase | Main idea | Common meaning | Example |
|---|---|---|---|
| by which | Method or means | by what method | The method by which we work |
| by which | Rule or standard | according to which | The rules by which they operate |
| by which | Deadline | no later than which | The date by which payment is due |
| in which | Place | where / inside which | The room in which we met |
| in which | Situation | in what situation | The situation in which he works |
| in which | Time | during which | The year in which she graduated |
| in which | Context | within which | The context in which the words were used |
Usage difference
Look at these two sentences:
This is the process by which the product is made.
The question is How is the product made?
Now compare:
This is the factory in which the product is made.
The question is Where is the product made?
That is the central difference.
Grammar logic
A useful way to check your sentence is to replace the phrase with a simple question.
If the answer is about how, think about by which.
If the answer is about where, in what situation, or during what period, think about in which.
For example:
The process ___ the data is analyzed is complicated.
Ask:
How is the data analyzed?
Answer: by a process.
So:
The process by which the data is analyzed is complicated.
Now:
The laboratory ___ the data is analyzed is secure.
Ask:
Where is the data analyzed?
Answer: in the laboratory.
So:
The laboratory in which the data is analyzed is secure.
Sentence structure difference
Both phrases can follow a noun and introduce a relative clause:
Noun + by which + clause
the method by which we solve the problem
Noun + in which + clause
the room in which we solve the problem
The main difference is not the structure. The main difference is the preposition and the meaning it creates.
Cambridge notes that which can work with prepositions, and the preposition can appear directly before which in more formal writing or at the end of the relative clause in less formal English.
Compare:
the method by which we solved it
with the less formal:
the method which we solved it by.
The first version sounds more formal and polished.
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Grammar Rules You Must Remember
Rule 1: Use “by which” for method or means
When the sentence answers the question “How?”, by which is often correct.
This is the method by which the company trains new employees.
The method explains how the training is done.
Rule 2: Use “in which” for place or situation
When the sentence answers “Where?” or describes a situation, in which is often the right choice.
This is the office in which she works.
You can usually replace this with:
This is the office where she works.
Rule 3: “By which” can show a deadline
This use is easy to miss.
Please submit the form by Friday.
A more formal version is:
Friday is the date by which you must submit the form.
Here, by which means “no later than that date.”
This is not about a method. It is about a time limit.
Rule 4: “In which” can refer to time or a period
We can use in which with words such as year, month, period, decade, or era when the meaning is “during that time.”
2010 was the year in which the company opened its first branch.
This means:
2010 was the year during which the company opened its first branch.
Common Mistakes Students Make
Learners often make mistakes because they translate directly from their first language or choose the preposition based only on the noun.
Mistake 1: Using “in which” for a method
Wrong:
This is the way in which the machine works.
This sentence is actually possible in English, but learners sometimes use it when they really mean a clear method and then mix the prepositions in other structures.
A safer distinction is:
This is the method by which the machine works.
For “way,” both in which and by which may occur depending on the exact meaning and style, so do not treat every noun as having only one possible preposition.
Mistake 2: Using “by which” for a place
Wrong for the intended place meaning:
This is the room by which we had lunch.
Correct:
This is the room in which we had lunch.
Why? Because lunch happened in the room, not by means of the room.
Mistake 3: Adding an extra pronoun
Wrong:
This is the office in which she works there.
Correct:
This is the office in which she works.
The relative clause already connects the office with the sentence. An extra there is unnecessary.
This is a common relative-clause problem. Cambridge also warns learners not to add an extra pronoun when the relative structure already performs that job.
Mistake 4: Choosing the preposition without checking the verb
Before using by which or in which, think about the original relationship.
Ask:
Did the action happen in something?
Use in.
Ask:
Was something done by means of something?
Use by.
This small habit can prevent many mistakes.
Easy Trick to Remember the Difference
Here is a simple classroom trick.
Think: “BY = HOW”
When you see by which, mentally ask:
How? By what method?
For example:
The method by which we communicate.
Ask:
How do we communicate?
Answer:
By this method.
Think: “IN = WHERE / WITHIN”
When you see in which, ask:
Where? Inside what? In what situation?
For example:
The room in which we study.
Ask:
Where do we study?
Answer:
In the room.
This trick is not perfect for every possible use because prepositions can have several meanings. Still, it is excellent for beginners and works in many common sentences.
You can also remember:
BY → method, means, rule, deadline
IN → place, situation, period, context
That four-word pattern is worth memorizing.
Daily Life Examples
These phrases can sound formal, but they are useful in everyday English too.
1. Talking about a method
Do you understand the way by which this app works?
A more casual version would be:
Do you understand how this app works?
2. Talking about a classroom
This is the classroom in which I studied English.
More natural in everyday speech:
This is the classroom where I studied English.
3. Talking about a process
The teacher explained the process by which the marks are calculated.
This means the teacher explained how the marks are calculated.
4. Talking about a situation
Nobody expected the situation in which we found ourselves.
The phrase describes the situation surrounding the people.
5. Talking about a deadline
Monday is the date by which you must pay the fee.
This means payment must happen on or before Monday.
6. Talking about a year
2024 was the year in which I started learning English.
The meaning is “the year during which.”
7. Talking about rules
These are the rules by which the competition is judged.
The rules explain the standard used for judging.
8. Talking about a place
The restaurant in which we ate was very busy.
In normal conversation, most speakers would probably say:
The restaurant where we ate was very busy.
9. Talking about a system
They created a system by which customers can track their orders.
The system is the means that makes tracking possible.
10. Talking about a context
We need to understand the context in which he made that comment.
The comment happened within a particular situation or background.
These examples show an important point: by which and in which are not simply formal versions of each other. They express different relationships.
Practice Section
Choose by which or in which to complete each sentence.
1.
This is the method ______ we use to solve the problem.
2.
That is the room ______ the interview took place.
3.
Friday is the day ______ you must submit the application.
4.
She explained the process ______ the machine produces clean water.
5.
I remember the year ______ we moved to this city.
Answers
1. by which
This is the method by which we use to solve the problem.
2. in which
That is the room in which the interview took place.
3. by which
Friday is the day by which you must submit the application.
4. by which
She explained the process by which the machine produces clean water.
5. in which
I remember the year in which we moved to this city.
A useful practice habit is to ask yourself one question before choosing the phrase: Am I talking about how something happens, or where/when/within what situation it happens?
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FAQs
What is the difference between “by which” and “in which”?
By which usually expresses a method, means, rule, standard, or deadline. In which commonly expresses a place, situation, period, or context.
For example, the method by which we work focuses on how, while the room in which we work focuses on where.
Can we use “by which” in questions?
Yes. It is possible, especially in formal English.
For example:
By which method did you solve the problem?
However, everyday speakers may simply ask:
How did you solve the problem?
The formal version is common in academic, technical, or careful writing.
Can we use “in which” in questions?
Yes.
For example:
In which room will the meeting take place?
This is grammatical and fairly formal. In ordinary conversation, people often say:
Which room will the meeting be in?
Both forms are understandable, but the word order is different.
Is “by which” formal or informal?
By which is generally more formal than everyday alternatives such as how or a preposition placed at the end of a sentence.
For example:
The method by which the system operates is complex.
More conversational:
The method the system operates by is complex.
Formal writing often places the preposition directly before which.
Is “in which” formal or informal?
In which can sound formal, especially when where would be more natural in conversation.
For example:
The house in which I grew up is still there.
More conversational:
The house where I grew up is still there.
Both are correct. The choice often depends on style, tone, and the type of writing.
Can “in which” be replaced by “where”?
Often, yes, when the noun refers to a place.
The city in which I live is busy.
The city where I live is busy.
However, in which is not limited to physical places. It can also refer to situations, periods, and contexts, so where is not always a complete replacement. Relative clauses with where and which follow specific grammatical patterns.
Conclusion
The difference between by which and in which becomes much easier when you stop looking at the words separately and focus on the relationship they express.
Use by which when you are talking about a method, means, rule, process, standard, or deadline. Think about the question “How?” or, in the deadline case, “By what time?”
Use in which when you are talking about a place, situation, period, environment, or context. Think about “Where?”, “In what situation?”, or “During what time?”
Remember these simple patterns:
The method by which… → how something happens
The room in which… → where something happens
The date by which… → the deadline
The year in which… → the period when something happened
You do not need to memorize hundreds of difficult examples. Start by identifying the meaning of the preposition.
Then choose the phrase that matches that meaning. With a little practice, these expressions will feel natural rather than confusing.
Relative clauses are a useful way to connect ideas clearly and avoid unnecessary repetition in English.
Liam Derren is a language writer at GrammarLamp, creating simple, helpful content about grammar, words, and everyday English. He makes language learning clear, practical, and easy to understand.