By Which or In Which helps English learners, writers, and students understand how relative phrases connect clauses and show different relationships in a sentence.
By which explains a method, means, or process used to do something, while in which describes a place, time, situation, location, or context where something exists or happens. The key difference between by which and in which depends on the verbs and prepositions a sentence requires. For example, method by which we solved the problem explains how something was done, while town in which I grew up explains where something happened. In my experience, this simple choice helps you choose correctly and avoid common mistakes every time. You normally use by with expressions such as achieved by and caused by, while in appears in occurred in and lived in. You cannot swap them freely because each preposition changes the meaning and function of the relative clause.
When using these expressions, consider whether you are explaining a method or process or describing a place time or situation. By which talks about the method involved in doing something, while in which tells where or when an event takes place. A clear example can help readers understand and convey ideas with better grammar, writing, and clarity. I once wrote a letter about a city and a condition that occurs there, which showed me how useful context can be. Both forms answer questions, but by which asks about the method, whereas in which asks about the setting or circumstances.
This approach also works when explaining lessons, stories, and essays to children or students who learn correct lines for school work. It helps sentences sound proper and complete, while supporting confidence and helping learners master formal, academic writing, formal writing, and professional communication. These forms fit well across education and communication, helping teachers and writers make the right usage choice and understand the relationship between their ideas.
By Which or In Which: The Main Difference
The simplest way to understand by which or in which is to focus on the prepositions by and in.
By commonly expresses a method or means. In commonly expresses location, inclusion, circumstances, environment, or a period. Therefore, the choice depends on the idea you want to communicate.
| Expression | Main meaning | Question it often answers | Example |
| by which | Method, means, process, mechanism | How? By what means? | “The process by which the material is produced” |
| in which | Place, situation, context, environment, period | Where? Within what? During what? | “The environment in which the material is produced” |
Consider this pair:
“They explained the process by which the device converts heat into energy.”
Here, by which introduces the process or mechanism.
Now compare:
“They studied the environment in which the device operates.”
Here, in which identifies the environment.
The distinction becomes even clearer when you replace the phrases with simpler wording:
- by which → by the method that, through which, or sometimes how
- in which → where, within which, or sometimes during which
These replacements aren’t always exact. Still, they provide a useful mental shortcut.
What Does “By Which” Mean?
By which refers to the method, means, process, or mechanism through which something happens.
The word by often answers a question about the way something occurs. For example, if someone asks, “How was the problem solved?” the answer might describe the method used.
“This is the method by which the problem was solved.”
The phrase by which connects method with the action that follows.
You will often see this construction after nouns such as:
- method
- means
- process
- procedure
- system
- mechanism
- technique
- approach
- way
- principle
For example:
- “Researchers examined the process by which cells divide.”
- “The company changed the system by which customers submit applications.”
- “This is the technique by which the material is tested.”
- “They described the mechanism by which the lock works.”
In each sentence, the relative clause explains how something happens.
By Which for Methods
One of the most common uses of by which involves a method.
“This is the method by which the data was collected.”
The clause by which the data was collected explains the method.
You could also write:
“This is how the data was collected.”
The second version sounds more direct and conversational. The first version fits naturally when the sentence needs to identify or describe a specific method.
By Which for Processes
By which also works well when describing a process.
“Scientists studied the process by which plants convert light into chemical energy.”
The phrase doesn’t identify a physical location. Instead, it explains the process responsible for the result.
Another example:
“The article explains the process by which a bill becomes law.”
Here, by which connects the noun process to the series of actions that follows.
By Which for Means
The expression can also introduce the means used to achieve an outcome.
“They developed a system by which customers could track their orders.”
The system provides the means through which customers track orders.
Another example:
“The organization created a platform by which members could exchange information.”
The platform isn’t necessarily a physical place. It functions as a means or mechanism.
By Which in Formal and Academic Writing
You will encounter by which frequently in academic, scientific, technical, and professional writing because it can express complex relationships precisely.
For example:
“The researchers investigated the mechanism by which bacteria resist antibiotics.”
This wording clearly connects the mechanism with the result being studied.
However, formal doesn’t automatically mean better. If how communicates the same idea more naturally, it may be the better choice in conversational writing.
Compare:
“The report explains the manner by which the system operates.”
with:
“The report explains how the system operates.”
The second sentence is shorter and more natural for many readers.
What Does “In Which” Mean?
In which generally refers to a place, situation, environment, context, condition, or period in which something happens.
For example:
“This is the room in which the meeting took place.”
The phrase tells you where the meeting took place.
But in which isn’t limited to physical locations. English also uses it for abstract situations and periods.
For example:
“They entered a period in which prices increased rapidly.”
Here, period isn’t a physical location. In which means something like during that period.
Similarly:
“She faced a situation in which she had to choose between two options.”
The phrase identifies the circumstances surrounding the action.
In Which for Physical Places
A straightforward use of in which describes a physical place or container.
“The building in which they work has six floors.”
You can often replace in which with where:
“The building where they work has six floors.”
Both are grammatical. Where usually sounds more natural in everyday conversation, while in which can sound more formal.
Other examples include:
- “The room in which we met was small.”
- “The house in which she grew up has been renovated.”
- “The laboratory in which the samples were tested is secure.”
In Which for Situations
English frequently uses in which with abstract nouns.
“They found themselves in a situation in which every option carried a risk.”
The sentence doesn’t describe a room or physical container. Instead, in which introduces the circumstances surrounding the situation.
You can find the same pattern with words such as:
- situation
- circumstance
- condition
- context
- case
- scenario
- environment
- setting
For example:
“The policy created conditions in which small businesses could grow.”
The businesses aren’t literally inside the conditions. The phrase describes the circumstances under which growth occurred.
In Which for Time Periods
In which can also identify a particular period.
“It was a year in which the company expanded rapidly.”
Here, in which has a time-related meaning. You could paraphrase it as:
“It was a year during which the company expanded rapidly.”
Another example:
“The decade in which the technology emerged changed the industry.”
In this context, in which points to a period rather than a physical location.
By Which or In Which: A Quick Comparison
The easiest practical distinction is this:
By which usually describes how something happens. In which usually describes where, within what situation, or during what period something happens.
| Sentence | Why it works |
| “The method by which the test is performed” | Describes how the test is performed |
| “The laboratory in which the test is performed” | Describes where the test is performed |
| “The process by which the material changes” | Describes the mechanism |
| “The environment in which the material changes” | Describes the surrounding conditions |
| “The system by which payments are processed” | Describes the operating method |
| “The system in which payments are recorded” | Describes the system containing the records |
This comparison reveals an important point: the same general topic can use either expression when the relationship changes.
A Useful Question Test
When you’re unsure, ask yourself:
Am I explaining how something happens?
If yes, by which may be appropriate.
“The process by which the product is manufactured.”
Am I identifying where, within what situation, or during what period something happens?
If yes, in which may be appropriate.
“The factory in which the product is manufactured.”
This test works remarkably well in everyday writing.
By Which vs. In Which With Similar Nouns
Some mistakes happen because writers assume a particular noun always takes one expression. That’s not how these constructions work.
Consider system:
“The system by which payments are processed.”
This focuses on the method or mechanism.
Now consider:
“The system in which payment records are stored.”
This focuses on the environment or framework containing the records.
The noun system stayed the same. The relationship changed.
The same thing happens with process and stage:
“The process by which the product is manufactured.”
This explains how manufacturing occurs.
“The stage in which the product is tested.”
This identifies a particular part of the process where or during which testing occurs.
That distinction is more useful than trying to memorize fixed combinations.
By Which vs. In Which vs. Which
The word which can function as a relative pronoun, but the preposition may carry important meaning.
Compare:
“The method by which the results were calculated.”
The phrase explains the method.
Now remove by:
“The method which the results were calculated.”
That sentence doesn’t express the intended relationship correctly. The verb phrase were calculated needs the appropriate relationship to method.
Similarly:
“The room in which the meeting occurred.”
Removing in gives:
“The room which the meeting occurred.”
That doesn’t work because occurred doesn’t connect directly to room in that structure.
The preposition isn’t decorative. It tells the reader how the noun relates to the following clause.
By Which vs. In Which vs. Where
Where is one of the most useful alternatives to in which when the antecedent refers to a place.
Compare:
“The restaurant in which we ate was crowded.”
“The restaurant where we ate was crowded.”
Both sentences are grammatical.
The second version is usually more natural in ordinary conversation. The first can sound more formal or deliberate.
However, where cannot simply replace by which.
For example:
“This is the method by which the machine operates.”
You can’t naturally turn that into:
“This is the method where the machine operates.”
The meaning changes because where expresses location or setting. It doesn’t normally express the method itself.
When “Where” Works Naturally
Use where when the antecedent refers to a location or setting:
- “the house where I grew up”
- “the office where she works”
- “the city where they met”
- “the laboratory where the samples were tested”
In which can often replace where in these examples:
- “the house in which I grew up”
- “the office in which she works”
- “the city in which they met”
- “the laboratory in which the samples were tested”
By Which vs. In Which vs. Through Which
Another expression worth comparing is through which.
Through which often emphasizes a channel, passage, medium, or route.
Compare:
“The method by which the results were calculated.”
This focuses on the method.
Now:
“The platform through which customers submitted their applications.”
This emphasizes the platform as the channel used for submission.
The meanings can overlap, especially when describing systems or processes. Still, the prepositions aren’t interchangeable in every sentence.
Compare the Three
| Expression | Main focus | Example |
| by which | Method or means | “The method by which the data was analyzed” |
| in which | Place, context, situation, or period | “The environment in which the data was analyzed” |
| through which | Channel, passage, or medium | “The platform through which the data was submitted” |
Think of them as three different lenses. By highlights the means. In highlights the setting. Through highlights the channel.
Common Mistakes With By Which and In Which
Using “In Which” for a Method
A common error is using in which when the sentence needs by which.
Less appropriate:
“This is the method in which the machine operates.”
Better:
“This is the method by which the machine operates.”
The method answers how, so by which fits the intended relationship.
Read More: Too Fast vs. To Fast: What’s the Difference?
Using “By Which” for a Location
The reverse mistake happens when writers describe a location.
Less appropriate:
“This is the room by which the meeting took place.”
Better:
“This is the room in which the meeting took place.”
The room provides the setting. Therefore, in which works.
Choosing the Preposition by the Noun Alone
Don’t assume every occurrence of method, system, process, or place requires one fixed construction.
Look at the meaning.
For example:
“The system by which users authenticate their accounts.”
Here, the system represents a mechanism.
But:
“The system in which user records are stored.”
Here, the system represents an environment or framework.
The surrounding verb and intended relationship matter just as much as the noun.
Replacing Every “In Which” With “Where”
Where works beautifully with many places. However, it isn’t a universal substitute for in which.
Consider:
“The circumstances in which the decision was made were unusual.”
You could say:
“The circumstances where the decision was made were unusual.”
That version sounds awkward because circumstances aren’t being treated as a straightforward location.
A better alternative might be:
“The circumstances under which the decision was made were unusual.”
The correct preposition depends on the relationship.
Dropping the Required Preposition
Writers sometimes remove a preposition because which already appears in the sentence.
That can produce an incomplete relationship.
Compare:
“The process by which the software works.”
The preposition by connects the process to the way the software works.
Similarly:
“The environment in which the software operates.”
The preposition in connects the environment to the operation.
Removing the preposition without restructuring the sentence can make the grammar incorrect.
How to Choose Between By Which and In Which
When you’re editing a sentence, use this quick process.
Start With the Antecedent
Identify the noun that which refers to.
For example:
“The method by which the results were obtained.”
The antecedent is method.
Identify the Relationship
Next, ask what the relative clause tells you.
Does it explain a method or means?
“The method by which the results were obtained.”
Or does it describe a place, situation, environment, or period?
“The laboratory in which the results were obtained.”
Apply the Question Test
Use these questions as a quick guide:
- How? → often by which
- By what means? → often by which
- Where? → often in which
- Within what? → often in which
- During what period? → often in which
- Through what channel? → possibly through which
Check the Complete Sentence
Don’t make the decision from one word alone. Read the entire sentence.
For example:
“The system by which employees access the database has changed.”
The sentence discusses the mechanism of access.
But:
“The system in which employee records are stored has changed.”
Now the sentence discusses the environment or framework containing records.
That final check catches many errors.
Formality and Naturalness
Both by which and in which are standard English constructions. Neither expression is automatically better simply because it sounds formal.
In everyday writing, a shorter alternative may improve readability.
Compare:
“This is the method by which we solved the problem.”
with:
“This is how we solved the problem.”
The second version is more direct.
Likewise:
“This is the building in which she works.”
could become:
“This is the building where she works.”
The best choice depends on your audience, sentence structure, and desired level of formality.
Formal writing often needs precision. Casual writing often benefits from simplicity. Good writing balances both.
By Which and In Which in Academic Writing
Academic writing frequently uses by which and in which because researchers need to describe relationships precisely.
For example:
“The study examined the mechanism by which the treatment affected the cells.”
This sentence identifies the mechanism being investigated.
Another example:
“The researchers analyzed the conditions in which the samples remained stable.”
Here, in which describes the conditions surrounding the samples.
These constructions become particularly useful when a writer needs to connect a technical noun with a detailed relative clause.
However, academic writing doesn’t require complicated language at every opportunity. If how communicates the same idea clearly, it may be preferable.
By Which and In Which in Professional Writing
Professional documents use these constructions in policies, reports, technical documentation, and procedural explanations.
For example:
“The procedure by which employees submit reimbursement requests is outlined below.”
This tells readers which procedure controls the submission process.
Compare:
“The portal in which reimbursement records are stored is accessible to authorized employees.”
Now the sentence identifies the environment containing the records.
In business writing, clarity should come before formality. A polished sentence isn’t necessarily the one with the most elaborate wording.
The Grammar Behind By Which and In Which
To understand why these phrases work, break them into two parts:
- by + which
- in + which
Here, which is a relative pronoun. It refers back to a noun or noun phrase called the antecedent.
For example:
“The method by which the test works is complex.”
The antecedent is method.
The relative clause is:
by which the test works
The preposition by tells readers what relationship exists between method and the test works.
Now consider:
“The laboratory in which the test works is secure.”
The antecedent is laboratory.
The relative clause is:
in which the test works
The preposition in establishes the setting.
This is why replacing one preposition with another can alter the meaning significantly.
Can You End a Sentence With “By Which” or “In Which”?
English sometimes allows prepositions to appear at the end of a clause, especially in informal speech.
For example:
“Which system did you do it by?”
This is understandable and grammatical in ordinary English.
A more formal version might be:
“By which system did you do it?”
However, formal wording isn’t always more natural.
With in which, everyday English often prefers where:
“Which room did you meet in?”
This sounds natural in conversation.
A formal alternative is:
“In which room did you meet?”
The choice depends on context. Don’t treat sentence-final prepositions as automatically wrong. English grammar allows them in many constructions.
Is “By Which” Always Correct for “How”?
No. By which and how can overlap, but they don’t function identically in every structure.
Compare:
“The method by which the problem was solved.”
and:
“How the problem was solved.”
Both can communicate the manner of solving the problem. However, their grammatical structures differ.
By which normally relates back to an antecedent such as method.
How directly introduces the manner or process.
Another useful comparison:
“This is the process by which the material is purified.”
“This is how the material is purified.”
The second sentence is simpler. The first explicitly identifies the process.
Is “In Which” Always Correct for “Where”?
No. In which can often replace where, especially with physical places, but the two expressions aren’t interchangeable in every context.
For example:
“The office in which she works.”
and:
“The office where she works.”
Both work naturally.
But abstract nouns can require a different construction:
“The circumstances under which the agreement was signed.”
Replacing under which with where would weaken or distort the relationship.
This demonstrates a broader rule: choose the preposition based on meaning rather than appearance.
Quick Examples of By Which and In Which
The following examples show how a small preposition change can create a different relationship.
Method vs. Location
“The method by which the device operates is unusual.”
“The room in which the device operates is unusually cold.”
The first answers how. The second answers where.
Process vs. Stage
“The process by which the material is produced is automated.”
“The stage in which the material is tested takes two hours.”
The first describes a mechanism. The second identifies a point within a process.
System as Method vs. System as Environment
“The system by which payments are verified is secure.”
“The system in which payment records are stored is secure.”
The first emphasizes the system’s function as a mechanism. The second emphasizes the system as the environment containing records.
Time vs. Method
“The year in which the company was founded was difficult.”
“The method by which the company raised funds was unusual.”
The first identifies a time period. The second identifies a means.
A Practical Decision Table
| If you want to express… | Consider… | Example |
| A method | by which | “The method by which it works” |
| A means | by which | “The means by which funds were raised” |
| A mechanism | by which | “The mechanism by which the device functions” |
| A procedure | by which | “The procedure by which claims are processed” |
| A physical location | in which | “The room in which we met” |
| A situation | in which | “The situation in which they found themselves” |
| An environment | in which | “The environment in which the system operates” |
| A period | in which | “The year in which the event occurred” |
| A channel or medium | through which | “The platform through which users communicate” |
| A location in casual writing | where | “The office where she works” |
Practice: Choose “By Which” or “In Which”
Try choosing the correct expression before checking the answers.
- This is the method ___ the data was collected.
- The laboratory ___ the samples were analyzed is nearby.
- They explained the process ___ the machine converts heat into energy.
- It was a period ___ the company experienced rapid growth.
- The system ___ customers verify their identities has changed.
- The environment ___ the software operates requires specific hardware.
- The technique ___ the material is tested requires specialized equipment.
- The room ___ the equipment is stored must remain dry.
Answer Key
- by which — the phrase explains the method of collection.
- in which — the phrase identifies the laboratory where analysis occurred.
- by which — the phrase describes the mechanism or process.
- in which — the phrase identifies a period during which something happened.
- by which — the system functions as a method or mechanism.
- in which — the environment provides the setting.
- by which — the technique explains how testing occurs.
- in which — the room provides the physical location.
A Simple Memory Trick
If you regularly confuse by which and in which, remember this compact rule:
BY = the way. IN = the setting.
Think of a cooking example.
“The method by which the chef prepared the dish was unusual.”
That’s about how the chef prepared it.
Now:
“The kitchen in which the chef prepared the dish was small.”
That’s about where the preparation happened.
One describes the route to the result. The other describes the place or setting surrounding the action.
This mental picture makes the distinction easier to remember than a long grammar rule.
FAQs
What is the difference between by which and in which?
By which usually describes a method, means, or process, while in which describes a place, time, situation, or context. For example, “This is the method by which we solved it” explains how something happened. “This is the town in which I grew up” explains where it happened.
When should I use by which?
Use by which when you want to explain how something was done or identify the method used. It often connects with verbs or expressions that naturally take the preposition by, such as achieved by or caused by.
When should I use in which?
Use in which when you describe the location, time, setting, or circumstances of an event. For example, “The year in which she graduated was memorable” shows a specific time.
Can I replace by which with in which?
No, you generally cannot swap them freely because the preposition changes the meaning of the sentence. The correct choice depends on the verb, context, and relationship between the ideas.
Are by which and in which formal expressions?
Yes. Both are correct and can work well in formal writing, academic writing, and professional communication. They are especially useful when you want to create clear relative clauses and precise sentences.
Conclusion
Understanding by which and in which becomes easier when you focus on the relationship each phrase creates. By which points to a method, means, or process, while in which points to a place, time, situation, or context. The preposition required by the sentence usually gives you the answer. If you need to explain how something happened, by which may be appropriate. If you need to explain where, when, or under what circumstances something happened, in which is usually the right choice.
With regular practice, learners, students, and writers can use both expressions with greater clarity and confidence. Pay attention to the verb, preposition, and overall meaning before choosing between them. This simple habit can help you avoid common mistakes, improve your grammar and writing, and make your sentences sound more complete, formal, and natural.
