By which or in which If you have ever paused mid-sentence wondering whether to write “the method by which” or “the method in which,” you are not alone. These two phrases trip up even experienced writers. Both use the relative pronoun which, and both introduce relative clauses — yet they are not interchangeable. The preposition is everything.
In short: “by which” describes a method, means, or agency (how something happens), while “in which” describes a location, time period, or set of circumstances (where or when something happens). Choosing the wrong one does not just sound awkward — it changes the meaning of your sentence entirely.
This guide breaks down the difference clearly, with examples, a comparison table, common mistakes, and memory tricks to help you get it right every time.
By Which or In Which: What’s the Difference?
Both phrases are prepositional relative clauses. That means they combine a preposition (by or in) with the relative pronoun which to connect a noun to a descriptive clause. The key is that the preposition must match the verb in your sentence.
- “By which” answers the question How? — it points to a method, process, or means.
- “In which” answers the question Where? or When? — it points to a location, time, or situation.
Consider these two sentences side by side:
This is the process by which steel is hardened. This is the factory in which steel is hardened.
Same noun (process vs. factory), same relative pronoun, same verb — but the preposition flips the meaning completely. The first tells you how something happens; the second tells you where it happens.
Also read : Gluing vs. Glueing
When Do You Use “By Which”?
Use “by which” when the verb in your sentence naturally pairs with by. This typically happens when you are describing a method, mechanism, standard, or means of doing something.
Common verb phrases that work with “by”:
- achieved by, accomplished by
- caused by, triggered by
- measured by, judged by, evaluated by
- defined by, determined by
- made by, produced by
Examples of correct “by which” usage:
- The process by which water is purified involves multiple filtration stages.
- The criteria by which candidates are evaluated include experience and communication skills.
- She didn’t understand the mechanism by which the drug was supposed to work.
- That is the system by which all decisions are made.
Quick test: Replace “by which” with “how.” If the sentence still makes sense, “by which” is likely correct.
The process by which water is purified… → The process [how] water is purified… ✓
When Should You Use “In Which”?
Use “in which” when your verb naturally takes in — usually to describe a physical place, a span of time, a situation, or a set of circumstances.
Common verb phrases that work with “in”:
- occur in, happen in, take place in
- live in, work in, study in
- be found in, be located in
- be based in, be set in
Examples of correct “in which” usage:
- The country in which she was born no longer exists.
- That was the year in which everything changed.
- The circumstances in which the decision was made were extremely difficult.
- He described the case in which the defendant had no legal representation.
Quick test: Replace “in which” with “where” or “when.” If the sentence holds, “in which” is the right call.
The year in which everything changed… → The year [when] everything changed… ✓
How Do Prepositions Change Meaning?
Prepositions are small words, but they carry enormous weight. They signal the relationship between a noun and the action in a clause. When you swap by for in, you shift the entire focus of the sentence.
| Sentence | Preposition | What It Expresses |
|---|---|---|
| The method by which we track progress | by | How progress is tracked (method) |
| The system in which we track progress | in | Where/context of tracking (situation) |
| The means by which he escaped | by | How he escaped (means) |
| The building in which he escaped | in | Where he escaped from (location) |
| The standard by which success is measured | by | How success is measured (criterion) |
| The industry in which success is measured | in | The field or context (situation) |
Notice how every time you change by to in, the question being answered shifts from How? to Where/In what context?
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Examples Showing Both Phrases
Correct Usage Examples
These sentences use the right preposition for the verb and meaning involved:
- The algorithm by which search results are ranked has changed significantly. (method)
- The region in which the earthquake occurred was densely populated. (location)
- Scientists are studying the process by which cancer cells replicate. (mechanism)
- The decade in which she wrote her best work was the 1980s. (time period)
- The scale by which we measure economic growth is GDP. (standard/criterion)
- The environment in which children learn shapes their development. (circumstances)
Incorrect Usage Examples
These sentences use the wrong preposition — and the meaning breaks down:
| Incorrect | Why It’s Wrong | Correct |
|---|---|---|
| The method in which they solved it was brilliant. | You solve something “by” a method, not “in” it | The method by which they solved it was brilliant. |
| The country by which she was raised values education. | You are raised “in” a country, not “by” it | The country in which she was raised values education. |
| The process in which data is encrypted must be updated. | Encryption happens “by” a process | The process by which data is encrypted must be updated. |
Context Variations
Sometimes the same noun can appear with either phrase, depending on the intended meaning:
- The framework by which the law operates (how the law works)
- The framework in which the law operates (the system or context surrounding the law)
Both can be grammatically correct — the choice depends entirely on what you want to communicate.
Common Mistakes with Relative Clauses
Even confident writers make these errors. Knowing them upfront saves you from embarrassment in academic papers, business writing, or formal emails.
Mistake 1: Mixing up the prepositions As shown above, swapping by and in distorts your meaning. Always ask: does my verb take by or in?
Mistake 2: Using “which” alone when a preposition is needed
- Incorrect: The method which he used was unconventional.
- Correct: The method by which he used… wait — “by which he used” doesn’t work either.
- Better: The method he used was unconventional. (Drop “which” when it’s the object and no preposition is needed.)
3: Adding a redundant preposition at the end In formal writing, the preposition goes before which — not at the end:
- Informal (acceptable): The place which she grew up in.
- Formal (preferred): The place in which she grew up.
Mistake 4: Using “that” instead of “which” after a preposition You cannot say “by that” or “in that” in a prepositional relative clause. Which is required after a preposition.
- Incorrect: The method by that the vote was counted…
- Correct: The method by which the vote was counted…
Also read : Deem Fit — What It Means, How To Use It, And Why It Matters
Memory Tricks for Choosing Correctly
Struggling to remember which phrase to use? These simple strategies make the choice automatic.
Trick 1: The substitution test
- Swap “by which” with how → if it sounds natural, use “by which”
- Swap “in which” with where or when → if it sounds natural, use “in which”
2: The verb test Look at the verb in your clause and ask: what preposition do I normally use with it?
- “Measured ___” → measured by → use by which
- “Located ___” → located in → use in which
Trick 3: The question trick
| If your sentence answers… | Use… |
|---|---|
| How? (method, means, process) | by which |
| Where? (place, location, setting) | in which |
| When? (year, era, period) | in which |
| In what circumstances? | in which |
4: The formality indicator Both “by which” and “in which” belong to formal, written English. In casual speech, most people rephrase entirely: “the way we do it” instead of “the method by which we do it,” or “where I grew up” instead of “the place in which I grew up.” If you are writing formally — reports, essays, academic papers — these phrases add precision and polish.
Conclusion
The difference between “by which” or “in which” comes down to one small but powerful word: the preposition. By signals method, means, or agency it answers how. In signals location, time, or circumstance it answers where or when.
When you face the choice, just run the substitution test. Try swapping the phrase with how or where/when and see which one fits. Then match the preposition to your verb and you will get it right every time.
These phrases are hallmarks of careful, formal writing. Using them correctly shows grammatical confidence and that is something readers and editors notice.
FAQs
Can “by which” and “in which” ever be used interchangeably?
No. They have different meanings — swapping them changes what the sentence communicates. Always match the preposition to the verb.
Is “in which” more formal than just using “where”?
Yes. “In which” is the formal equivalent of “where” in relative clauses, preferred in academic and professional writing.
Can I use “that” instead of “by which” or “in which”?
No. After a preposition, you must use which, not that. “By that” and “in that” are not valid in prepositional relative clauses.
When should I use “through which” instead of “by which”?
Use “through which” when emphasizing passage, transmission, or a channel — for example, “the pipe through which water flows.” Use “by which” for method or means more broadly.
Is it grammatically wrong to put the preposition at the end of the clause?
Not wrong — it’s simply less formal. “The place she grew up in” is acceptable in everyday speech. “The place in which she grew up” is preferred in formal writing.
Does “in which” only refer to physical places?
No. It can refer to time periods (“the year in which”), abstract situations (“the circumstances in which”), and conditions (“the state in which”) as well.
How do I know which preposition a verb takes?
Practice and reading help, but a quick tip: ask yourself what word you use after the verb naturally. “Judged by” → by which. “Situated in” → in which.
