When we talk about “by which” or “in which”, we’re discussing phrases that appear often in formal writing, such as contracts, research papers, essays, and business emails. These expressions, though similar, serve distinct purposes. In simpler terms, “by which” is usually about the means or method of something, while “in which” refers to the specific context or setting of an event. For example, a study might describe a process by which it was conducted, while an essay might detail a scenario in which an idea was tested. These small differences in meaning help you communicate more effectively.
Mastering these expressions requires an understanding of their subtle distinctions. Clear rules and helpful comparisons can guide you in knowing when to use each. Consider this: a contract might state, “the policy by which we operate,” referring to a method, while a report might say, “the project in which we participated,” referring to a specific situation. The key lies in context, meaning, and syntax. By following the right rules, you ensure proper usage in both formal contexts and professional communication, making your writing clearer and more purpose-driven.
Why “By Which” vs “In Which” Confuses So Many Writers
Let’s start with the big picture.
English has tons of relative clauses – phrases like “in which,” “by which,” “for which,” and more. These help link ideas in complex sentences. But because they’re formal and rely on prepositions, even native speakers stumble on them.
Here’s what makes it tricky:
- They look similar but have different meanings.
- Prepositions affect meaning heavily.
- Some phrases feel more natural in speech (where), while others suit writing (in which).
- Word placement can change the tone or clarity of your sentence.
That’s why we’re breaking this down – with clarity, simplicity, and purpose.
How Relative Clauses Work (And Why It Matters Here)
Before diving into “by which” and “in which,” it’s smart to understand how relative clauses function in English.
What is a Relative Clause?
A relative clause gives extra information about a noun. It starts with a relative pronoun like:
- which
- who
- that
- where
- when
- whom
- whose
Now let’s plug in the prepositions that change everything.
Prepositions + “Which” = Precision
When you combine a preposition with “which,” you get powerful, precise phrases like:
| Phrase | Function | Example |
| in which | Indicates location or context | The folder in which the files are stored |
| by which | Shows method, cause, or means | The process by which water is purified |
| through which | Indicates a path or process | The channel through which money flowed |
These phrases add clarity, but they need to be used in the right context. Let’s explore each in detail.
What Does “In Which” Really Mean?
“In which” is used when something is inside, part of, or occurs within another thing – whether literal or abstract.
Use Cases of “In Which”
- Physical location The drawer in which I keep my keys is locked.
- Time/abstract concept The year in which she was born was historic.
- Situational context The meeting in which we discussed the proposal was tense.
Think of it this way: If you can say “in the ___,” then “in which” likely works.
Breakdown of the Phrase
- “In” is the preposition (shows position)
- “Which” refers to the noun it modifies
text
CopyEdit
This is the house [in which I grew up].
Or in more natural speech:
text
CopyEdit
This is the house I grew up in.
Is “Where” a Better Option?
Often, yes. In casual writing or speech, “where” is preferred.
| Formal | Informal |
| The city in which he lives | The city where he lives |
| The conditions in which they worked | The conditions they worked under |
But for legal, academic, or technical writing, stick with “in which.”
What Does “By Which” Actually Mean?
“By which” expresses method, cause, or agency – how something happens.
Use Cases of “By Which”
- Method or process The system by which data is encrypted is secure.
- Cause or consequence The act by which the contract was voided was illegal.
- Means or tool The key by which the safe was opened is lost.
How to Identify It
If you’re explaining how something is done, “by which” is your friend.
| Function | Example |
| Describing method | The process by which vaccines are developed |
| Describing cause | The rule by which he was disqualified is outdated |
| Describing tool | The code by which the door opens was stolen |
“Where,” “Wherein,” and “In Which”: Know the Difference
“Where”
- Used casually
- Refers to location
- Works for both physical and abstract places
Example: The house where I was born.
“In Which”
- More formal
- Used in academic or technical writing
- Refers to abstract or formal locations
Example: The document in which that law is recorded.
“Wherein”
- Extremely formal or old-fashioned
- Can mean in which or during which
Example: An agreement wherein both parties benefit.
Quick Comparison Table
| Phrase | Style | Meaning | Example |
| Where | Informal | Location | The room where I slept |
| In which | Formal | Location/Context | The room in which I slept |
| Wherein | Very formal | Inside/during a situation | The clause wherein the law applies |
“By Which” vs “In Which” – Side-by-Side Examples
Let’s look at some real comparisons.
| Sentence with “By Which” | Sentence with “In Which” |
| The method by which we solve this is flawed. | The case in which that method was used is under review. |
| The law by which he was judged is outdated. | The court in which the ruling occurred was corrupt. |
| A process by which we ensure quality. | The phase in which testing occurs is vital. |
✅ Tip: If it answers “how?”, it’s likely “by which.” ✅ If it answers “where?” or “in what?”, it’s “in which.”
When to Use Each Based on Context
Use “In Which” When:
- Describing time, place, or situation
- Referring to documents, chapters, sections
- Writing formally or academically
Examples:
- The year in which the reform passed was chaotic.
- The folder in which this document resides is confidential.
Use “By Which” When:
- Explaining how something is done
- Describing means or methods
- Citing cause or reasoning in logical writing
Examples:
- The means by which this decision was made is flawed.
- The standard by which we judge performance changed.
Common Pitfalls to Avoid
Overusing “In Which”
❌ Incorrect: He lives in a city in which there is a school in which he studied. ✅ Better: He lives in a city where he studied at school.
Misusing “By Which” for Location
❌ Incorrect: The building by which the event occurred… ✅ Correct: The building in which the event occurred…
Dangling Prepositions (in formal writing)
Informal: The box that I put it in Formal: The box in which I put it
Write Better Sentences Using “In Which” and “By Which”
Want to clean up your writing? Follow these steps:
Sentence Restructuring Tips
- Use “where” in informal writing unless precision is required.
- If your sentence feels clunky, try moving the preposition to the front.
- Don’t double up phrases like “in which… in which…”
Quick Grammar Check: Can You Spot the Right Phrase?
Try these. Fill in the blank with “by which” or “in which.”
- The law ___ the decision was made is under debate.
- The box ___ the keys are stored is locked.
- The method ___ we measure growth has evolved.
- The house ___ she was raised is now a museum.
Answers:
- by which
- in which
- by which
- in which
Memory Shortcuts and Rules of Thumb
- If it’s location or context, use in which
- If it’s method, cause, or means, use by which
- If unsure, replace it with “how” or “where” – does it still make sense?
Other Related Phrases Worth Knowing
Here are more “preposition + which” phrases you’ll encounter:
| Phrase | Function | Example |
| through which | Path or process | The pipe through which water flows |
| under which | Conditions | The rules under which we operate |
| for which | Purpose or benefit | The reason for which he applied |
| with which | Association or tool | The tool with which he fixed it |
| to which | Direction or reference | The document to which she referred |
Final Thoughts
Mastering the distinction between “by which” and “in which” might seem small, but it can have a powerful effect on how polished and professional your writing appears. Whether you’re crafting academic essays, business documents, legal contracts, or polished blog posts, the right phrase adds precision and credibility to your message.
“In which” is your go-to phrase when discussing something occurring within a place, situation, or abstract concept. It’s structured, formal, and ideal for writing that demands clarity. On the other hand, “by which” speaks to process, means, or method – how something happens or is caused.
What does “by which” mean in formal writing?
“By which” is used to describe a method, means, or cause. It answers the question “how?” or “by what method?” and is common in academic, legal, and scientific writing. For example: The process by which oxygen is produced in photosynthesis is well-studied.
How do I decide between “in which” and “where”?
Use “where” in casual, everyday language. Use “in which” when writing formally, especially in academic or legal contexts. They often mean the same, but “in which” adds formality and precision. Example: The office in which he works vs The office where he works.
Is it incorrect to end a sentence with a preposition like “in”?
In informal English, it’s totally fine to end a sentence with a preposition. But in formal writing, we often move the preposition in front of the relative pronoun – creating phrases like “in which.” This avoids ending the sentence with a dangling preposition.
Can “by which” be used to describe location?
No, “by which” refers to method or cause – not physical location. If you’re talking about where something happens or exists, use “in which” or “where.” For example, The room in which the event occurred is correct – not by which.
Why is “in which” more common in formal writing?
“In which” is preferred in formal writing because it’s grammatically precise and avoids casual phrasing. It helps ensure clarity and structure, especially in legal, academic, or technical documents where ambiguity is risky or confusing.