English learners often get confused by by which or in which because both expressions contain the same relative pronoun, which. The small words by and in change the meaning of the whole phrase, so choosing the wrong one can make a sentence sound unnatural or even change its meaning By Which or In Which.
For example, compare these two sentences:
- This is the method by which we solved the problem.By Which or In Which.
- This is the room in which we had the meeting.
The first sentence talks about a method or means. The second talks about a place or situation. That simple difference is the key to understanding by which or in which.
You may see these expressions in school writing, formal emails, academic English, books, reports, and everyday speech. They are especially common when a sentence has a relative clause.
The good news is that you do not need to memorize hundreds of complicated rules. Once you understand what by and in mean in a sentence, the choice becomes much easier.By Which or In Which.
This guide explains by which or in which with clear grammar rules, examples, common mistakes, daily conversations, and practice questions. By the end, you should be able to choose the correct expression with much more confidence.By Which or In Which.
What Does “By Which” Mean?
By which is normally used when by shows the method, means, process, way, or instrument used to do something.By Which or In Which.
In simple words, by which often means:
- through which
- by means of which
- using which
- the method through which
For example:
This is the method by which the company checks its products.
Here, by which refers to the method. The company checks its products by means of that method.
This is the most important idea to remember when learning by which or in which.
When should you use “by which”?
Use by which when the noun before it is connected to a way of doing something.
Common nouns that can work with by which include:
- method
- way
- process
- system
- means
- procedure
- technique
- method of payment
- route
- mechanism
For example:
- This is the method by which we measure the temperature.By Which or In Which.
- She explained the process by which the machine works.
- That is the system by which students submit their assignments.
- We need to understand the way by which the problem was solved.
- The company changed the procedure by which employees apply for leave.
- This is the technique by which the artist creates the effect.
- They described the process by which water is cleaned.
- I finally understood the method by which the computer stores information.
Notice that by which connects an earlier noun with an action or process.
“By which” can sound formal
There is another important point about by which or in which. By which is grammatically correct, but it can sound formal.
In everyday English, native speakers often use that, which, or a phrase such as how instead.By Which or In Which.
For example:
This is the method by which we solved the problem.
This is formal and correct.
A more natural everyday version might be:
This is how we solved the problem.By Which or In Which.
Or:
This is the method we used to solve the problem.
So, by which is especially useful in formal writing, academic English, technical writing, and professional communication.
Common confusion with “by which”
Students sometimes choose by which simply because they see an action after it. That is not enough.
You should ask:
Does “by” show the method or means?
If yes, by which may be correct.
For example:
This is the process by which the information is collected.
The process tells us how the information is collected. Therefore, by which works.By Which or In Which.
What Does “In Which” Mean?
In which is generally used when in shows 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
This is an important part of understanding by which or in which.By Which or In Which.
For example:
This is the room in which we had our meeting.By Which or In Which.
Here, in which means in that room.By Which or In Which.
Another example:
I remember the year in which we first met.
Here, in which refers to a period of time. It means during that year.
When should you use “in which”?
Use in which when the noun before it represents a place, situation, condition, time period, or setting.
Examples include:
- This is the house in which I grew up.
- That was the year in which she started college.
- We entered a room in which several people were waiting.
- He described the situation in which the accident happened.
- This is the environment in which children learn best.
- I remember the period in which the business was struggling.
- They discussed the conditions in which the workers lived.
- She found the box in which her old photographs were stored.
In each sentence, in has its normal meaning of being inside, within, during, or within a particular situation.
“In which” and “where”
One useful way to understand in which is to compare it with where.
For example:
This is the house in which I was born.
You can also say:
This is the house where I was born.
Both are correct.
The version with where is usually more natural in everyday conversation. In which is more formal.
Similarly:
This is the office in which she works.
can become:
This is the office where she works.
Therefore, when thinking about by which or in which, remember that in which often has a close relationship with where.
Difference Between “By Which” and “In Which”
The easiest way to understand by which or in which is to focus on the meaning of the preposition.
By usually answers a question about method or means.
In usually answers a question about place, situation, or time.
| Expression | Main idea | Common meaning | Example |
|---|---|---|---|
| by which | Method or means | through which | This is the method by which we work. |
| in which | Place or situation | where/within which | This is the room in which we work. |
| by which | Process | by means of which | The process by which data is collected is simple. |
| in which | Time | during which | That was the year in which he graduated. |
| by which | System or technique | using which | The system by which payments are made is secure. |
| in which | Environment | within which | This is the environment in which children learn. |
The basic grammar logic
When choosing by which or in which, look at the noun before which.
If that noun describes a method, process, system, or means, by which is often appropriate.
This is the system by which the website protects user information.
The system explains how something happens.
If the noun describes a place, time, situation, or environment, in which is often appropriate.
This is the system in which the company operates.
Here, in which means within that system or situation.
The two expressions can sometimes appear with the same noun, but the meaning changes.
Consider the word system:
The system by which the company operates is efficient.
This focuses on the method or system used to operate.
But:
The system in which the company operates is competitive.
This focuses on the environment or structure within which the company operates.
That is why you should not simply memorize a list of nouns. You need to understand the relationship between the noun and the following clause.
“By which” = How?
A useful question is:
How did it happen?
If the answer is a method, by which may be the right choice.
This is the method by which we solved the problem.
How did we solve it?
By using this method.
“In which” = Where/Within what?
Ask:
Where did it happen?
Or:
Within what situation did it happen?
Then in which may be correct.
This is the room in which we solved the problem.
Where did we solve it?
In this room.
This simple question can help beginners understand by which or in which without memorizing difficult grammar terminology.
Grammar Rules You Must Remember
1. Use “by which” for a method or means
When by means “through” or “by means of,” use by which.
This is the method by which the problem was solved.
The method tells us how the problem was solved.
Another example:
They developed a system by which customers can pay online.
The system is the means that allows customers to pay.
2. Use “in which” for a place or location
When in means being inside a place, in which is normally appropriate.
This is the building in which my father works.
You could also say:
This is the building where my father works.
Both are grammatically correct, but where is more common in casual speech.
3. Use “in which” for periods of time
In which can refer to a year, month, period, or other time frame.
2020 was the year in which many people changed the way they worked.
Here, in which means during that year.
Another example:
She remembers the month in which she got her first job.
4. Use “in which” for situations and conditions
A situation can be something within which an event happens.
We found ourselves in a difficult situation in which nobody knew what to do.
Here, in which means in that situation.
This use is common in formal English.
5. “By which” often describes a process
A process explains how something is done or happens.
The process by which plants grow is fascinating.
The phrase describes the process through which growth occurs.
6. “In which” can often be replaced by “where”
When in which refers to a physical or figurative place, where may be easier.
This is the school in which I studied.
This is the school where I studied.
Both are correct.
This is especially useful when deciding between by which or in which in everyday conversation.
7. Do not choose the expression only because it sounds formal
Some learners think formal English always requires by which or in which. That is not true.
The preposition must match the meaning.
This is the room in which we met.
You cannot change it to by which simply to make the sentence sound more formal.
Common Mistakes Students Make
Mistake 1: Using “by which” for a place
Incorrect:
This is the office by which I work.
If you mean that you work inside the office, this is wrong.
Correct:
This is the office in which I work.
Or, more naturally:
This is the office where I work.
The mistake happens because students focus on which and forget the meaning of by.
Mistake 2: Using “in which” for a method
Incorrect:
This is the method in which we solved the problem.
If you mean the method used to solve the problem, by which is the better choice.
Correct:
This is the method by which we solved the problem.
An even simpler version is:
This is how we solved the problem.
Mistake 3: Thinking “which” decides the preposition
The word which does not decide whether you need by or in.
The meaning of the sentence decides.
Compare:
The method by which we work is simple.
and:
The environment in which we work is comfortable.
Both contain which, but the prepositions are different because the meanings are different.
Mistake 4: Forgetting that “where” can replace some uses of “in which”
Students sometimes believe that in which is always necessary in formal English.
Not so.
This is the city in which I was born.
Correct.
This is the city where I was born.
Also correct and usually more natural.
However, where cannot simply replace every use of by which.
This is the method by which we solved the problem.
You would normally say:
This is how we solved the problem.
Not:
This is the method where we solved the problem.
The word where is associated with place or context, not method.
Easy Trick to Remember the Difference
Here is a simple classroom trick for by which or in which.
Think: “BY = HOW”
Whenever by means the way something is done, think HOW.
The method by which we solved it…
Ask:
How did we solve it?
Answer:
By using this method.
So:
METHOD → BY WHICH
Think: “IN = WHERE”
When in refers to a place or situation, think WHERE.
The room in which we met…
Ask:
Where did we meet?
Answer:
In that room.
So:
PLACE → IN WHICH
You can also remember:
BY WHICH = through which
IN WHICH = where/within which
This trick is not a complete grammar rule for every possible sentence, but it works very well for common learner problems.
Another useful memory sentence is:
By which tells you how; in which tells you where or within what.
Keep that sentence in mind whenever you see by which or in which.
Daily Life Examples
Examples in Real Conversations
Although by which is more formal, you may see it in professional, academic, or technical conversations. In which is also common, especially in formal descriptions.
- This is the method by which we calculate the final price.A teacher might use this sentence while explaining mathematics or business.
- This is the room in which our team has its weekly meeting.In casual speech, someone would probably say, “This is the room where our team meets.”
- She explained the process by which the application is approved.Here, the process tells us how approval happens.
- I remember the year in which we moved to this city.Here, the phrase refers to a period of time.
- The company has a system by which customers can report problems.The system provides a way for customers to report problems.
- We discussed the situation in which the mistake happened.The mistake happened within a particular situation.
- He showed me the technique by which he fixes old watches.The technique explains how he does the work.
- They visited the village in which her grandparents lived.The village is a place.
- The process by which the machine operates is quite simple.The sentence focuses on how the machine works.
- That was a difficult period in which many families faced financial problems.
The phrase refers to a period of time and the situation within it.
These examples show why by which or in which cannot be selected randomly. Look at the relationship between the noun and the action that follows.
Practice Section
Choose the Correct Option
Choose by which or in which in each sentence.
1. This is the method ______ we use to clean the equipment.
A. by which
B. in which
2. This is the house ______ she was born.
A. by which
B. in which
3. They explained the process ______ the product is made.
A. by which
B. in which
4. I remember the year ______ we first met.
A. by which
B. in which
5. The system ______ payments are processed is secure.
A. by which
B. in which
Answers
1. A — by which
The method tells us how the equipment is cleaned.
2. B — in which
The house is a place, so in which is appropriate.
3. A — by which
The process explains how the product is made.
4. B — in which
A year is a period of time, so in which means during which.
5. A — by which
The system describes the means or method used to process payments.
A useful way to check your answer is to replace the expression with a simpler phrase.
If “how” works, think about by which.
If “where,” “inside,” “within,” or “during” works, think about in which.
FAQs
What is the difference between “by which” and “in which”?
By which normally refers to a method, means, process, or way of doing something. In which normally refers to a place, situation, condition, or period of time. For example, “the method by which we work” explains how, while “the room in which we work” explains where.
Can “in which” be used in questions?
Yes, but it is more common in formal questions or formal writing. For example: “In which room will the meeting take place?” In everyday speech, people may simply say, “Which room will the meeting be in?”
Is “by which” formal or informal?
By which is generally formal. You can find it in academic writing, official documents, reports, and professional English. In everyday conversation, speakers often use simpler expressions such as how, through which, or the way that.
Can “where” replace “in which”?
Often, yes, when in which refers to a place or setting. For example, “the school in which I studied” and “the school where I studied” are both correct. Where is usually more natural in everyday speech.
Can “where” replace “by which”?
Usually, no. By which often expresses a method or means, while where normally refers to a place or situation. Instead of “the method where we solved it,” say “the method by which we solved it” or, more naturally, “how we solved it.”
Is “by which or in which” always difficult to choose?
No. The main question is what the preposition means. If you are talking about how something is done, consider by which. If you are talking about where, within what, or during what period something happens, consider in which. This simple approach solves many common problems.
Conclusion
Understanding by which or in which becomes much easier when you stop looking only at the word which and focus on the meaning of the preposition.
Use by which when you are talking about a method, means, system, process, or way. It often has the meaning “through which” or “by means of which.”
Use in which when you are talking about a place, situation, environment, condition, or period of time. It can often mean “where,” “within which,” or “during which.”
Remember the easiest trick:
BY WHICH = HOW
IN WHICH = WHERE / WITHIN WHAT / DURING WHAT
For example:
This is the method by which we solved the problem.
But:
This is the room in which we solved the problem.
Both are correct because they express different ideas.
Do not worry if the difference feels confusing at first. Read real English, notice how these phrases are used, and practice making your own sentences. With a little practice, choosing between by which or in which will start to feel natural.
Hi, I’m Isabella Foster, an English language writer passionate about making grammar, vocabulary, and communication skills easy to understand. At EnglishVistor, I create clear, practical, and engaging learning resources to help learners improve their English with confidence.
