If you have ever paused mid sentence wondering whether to write “the method by which” or “the method in which,” you are not alone. The rule behind these phrases is simple once it clicks.
This guide breaks down when to use each phrase, with clear examples, a comparison table, and the idioms you will actually see in everyday English.
By Which vs In Which: What’s the Real Difference?
Both phrases combine a preposition with the relative pronoun “which” to connect a clause back to a noun. The preposition is what changes the meaning.
- By which points to the method, process, or means behind an action. It answers “how?”
- In which points to a place, time, or situation. It answers “where?” or “in what circumstance?”
A quick way to test this yourself is to drop the relative clause and check which preposition the base sentence actually needs.
- We solve problems by using a method, so we write “the method by which we solve problems.”
- We live in an era of technology, so we write “an era in which technology dominates.”
If the underlying verb naturally pairs with “by,” you need by which. If it pairs with “in,” you need in which. The verb decides the answer, not personal preference.
Use Which in a Sentence as a Conjunction
“Which” is classified as a relative pronoun rather than a true conjunction, but it functions the same way. It links a descriptive clause back to a noun without starting a new sentence.
Here is which working that way on its own, before any preposition gets added:
- The report, which took three weeks to finish, impressed the board.
- She bought a new laptop, which she uses for video editing.
- The bridge, which was built in 1932, is still standing today.
In each example, which connects extra information to the main clause smoothly. Add a preposition in front of it, such as by or in, and you get the phrases this article focuses on.
Which or In Which Grammar Examples
Choosing between a plain “which” and “in which” often comes down to whether a preposition is already doing work elsewhere in the sentence.
| Sentence type | Example | Why it works |
| Plain which | The car, which is red, belongs to my brother | No preposition needed |
| In which | The garage in which the car is parked is locked | In which sets the location |
| Plain which | The plan, which took months to build, finally launched | Straightforward relative clause |
| In which | The year in which the plan launched was difficult | In which marks the time frame |
A simple test: if you can comfortably ask “where” or “when” about the noun, in which usually fits better. If the sentence reads naturally with no preposition, plain which works fine.
When to Use “By Which” (Rules and Examples)
Use by which whenever you are explaining the method, system, or means behind something. It is common in academic, scientific, and legal writing that describes a process.
Verbs that typically pair with by which include achieve, measure, judge, solve, calculate, and determine.
Examples:
- The process by which plants convert sunlight into energy is called photosynthesis.
- The criteria by which applicants are evaluated include experience and education.
- The formula by which interest is calculated appears in the contract.
- Scientists are still studying the mechanism by which the virus spreads.
A helpful trick: if “through which” or “the way that” fits naturally in its place, by which is correct.
Which or In Which Examples
Sometimes the choice depends entirely on context rather than a fixed rule. Compare these pairs to see how the meaning shifts:
- The book, which I finished last night, was excellent. (simple description, no location)
- The chapter in which the twist happens is chapter twelve. (specific location within the book)
- The meeting, which ran long, covered three topics. (general comment)
- The room in which the meeting took place needed better lighting. (physical setting)
Plain which adds a side comment, while in which anchors the clause to a specific place, time, or scenario.
When to Use “In Which” (Rules and Examples)
Use in which when describing the place, time period, situation, or condition surrounding an event. It often replaces “where” in formal writing, especially when the noun is abstract rather than a literal location.
Common nouns that pair naturally with in which include situation, case, era, environment, circumstance, and scenario.
Examples
- The house in which she grew up was sold last year.
- We live in a time in which remote work has become standard.
- The situation in which the team found itself was stressful but manageable.
- This is a case in which the standard rules simply do not apply.
- The environment in which employees work directly affects productivity.
Because “where” cannot always replace in which, especially with abstract nouns like situation or system, in which is the safer choice in formal writing.
Common Expressions and Idioms Using By Which and In Which
Both phrases show up constantly in fixed expressions across legal writing, business reports, and academic papers. Knowing these patterns makes it easier to use them correctly without overthinking every sentence.
Expressions with “by which”
- The means by which
- The method by which
- The process by which
- The standard by which
- The extent by which (informal, often replaced with “to which”)
Expressions with “in which”
- The manner in which
- The way in which
- A world in which
- The case in which
- An instance in which
These set phrases are worth memorizing since they appear so often in professional and academic writing. Once they feel familiar, the right preposition becomes automatic.
Conclusion
The difference comes down to one simple test: are you describing how something happens, or where and when it happens? By which points to method and process. In which points to place, time, and situation.
Test your sentence without the relative clause and see which preposition the base verb naturally takes. You will almost always land on the correct phrase, and with practice this small grammar detail becomes second nature.
Brook is the creator and author behind Healthy Leeks, a platform focused on grammar, writing skills, and English language learning. Passionate about clear communication and effective writing, Brook shares practical grammar tips, easy-to-follow language guides, and educational content to help readers improve their English with confidence.
