If you have ever stared at a sentence and wondered whether to write by who or by whom, you are not alone. This is one of the trickiest grammar questions in the English language, often causing hesitation even for native speakers.
People confuse these phrases because traditional grammar rules clash with how modern, casual English is spoken every day. In this comprehensive guide, you will learn the exact difference between these two expressions, how grammar rules govern them, and when to use each one with total confidence.
In formal grammar, by whom is always the correct choice because “whom” is the objective pronoun required after a preposition. However, in casual, everyday conversation, native speakers almost always say by who. For professional writing, academic papers, and formal emails, you should always use by whom.
Which One Should You Use?
The short answer depends on how formal your writing needs to be. If you are writing a business report, a school essay, or a legal document, by whom is the only correct choice.
In casual text messages, phone calls, or relaxed conversations, by who is widely accepted because language naturally evolves toward simplicity. The golden rule is to match your choice to your audience and level of formality.
When in doubt, remember this takeaway sentence: Formality calls for whom, but conversation welcomes who.
Difference Table
| Feature | By Who | By Whom |
| Meaning | Asks the identity of the actor or agent | Asks the identity of the actor or agent |
| Usage | Casual speech, text messages, informal writing | Formal writing, academic papers, professional emails |
| Grammar | Technically violates traditional objective case rules | Follows traditional objective case rules strictly |
| Context | Conversational and relaxed settings | Professional, academic, and polite settings |
| Frequency | Very common in spoken daily English | Less common in speech, standard in print |
| Everyday Use | “Who was this cake made by?” | “By whom was this cake made?” |
Meaning of By Whom
The phrase by whom is a combination of the preposition “by” and the interrogative or relative pronoun “whom.” It is used to ask about the agent or doer of an action, typically in passive voice constructions.
Because “whom” functions as an object pronoun (just like him, her, or them), it fits perfectly after prepositions like by, to, with, or for.
Common Uses in Formal Contexts
- Asking about the creator of a work of art, a book, or a law.
- Inquiring about the sender of a formal letter or official email.
- Identifying the perpetrator in legal or investigative reports.
Everyday Situations
While rare in casual chat, you will encounter by whom in job interviews, official announcements, and literature. For example: “By whom was this policy enacted?” or “She is the author by whom we were inspired.”
Meaning of By Who
The phrase by who serves the exact same communicative purpose as its formal counterpart, but it breaks a traditional grammar rule by pairing a preposition with the subjective pronoun “who.”
In modern linguistics, this is viewed as a natural shift. Speakers prefer “who” because it feels more natural and requires less mental effort than pausing to recall objective case rules.
Where It Is Used
- Daily spoken conversations among friends and family.
- Modern media, TV shows, and podcasts.
- Informal online forums, social media posts, and quick chats.
Why It Is Considered Informal
Traditional grammarians classify “who” as a subject pronoun (used when the person is doing the action, as in “Who called you?”). When placed at the end of a passive sentence—such as “Who was the door locked by?”—it acts as the object of the preposition “by,” which strictly requires the objective form “whom.”
Why Do People Confuse These Words?
The confusion between these phrases stems from a collision between strict prescriptive grammar and descriptive everyday language use.
For centuries, English teachers have drilled objective pronoun rules into students. Yet, conversational English has heavily favored the word “who” for decades. Because native speakers use “who” in almost every other context, putting it at the end of a sentence feels completely natural to the human ear.
Grammar Rule
To master this choice, you need to understand the fundamental difference between subjective and objective pronouns.
- Subject Case (Who): Used when the person is performing the action.
- Object Case (Whom): Used when the person receives the action or follows a preposition.
| Pronoun Case | Pronoun Form | Example Sentence |
| Subjective | Who | Who wrote the letter? |
| Objective | Whom | To whom did you send it? |
Real-Life Examples
People
- Formal: “By whom was the award presented to the actor?”
- Informal: “Who was the award presented by?”
Technology
- Formal: “By whom was this software application developed?”
- Informal: “Who was this app built by?”
Business
- Formal: “By whom will the contract be signed tomorrow?”
- Informal: “Who is going to sign this contract?”
Common Mistakes
❌ Incorrect sentence: “By who was the Mona Lisa painted?”
✅ Correct sentence: “By whom was the Mona Lisa painted?”
Explanation: The preposition “by” requires the objective pronoun “whom” in formal writing.
❌ Incorrect sentence: “Whom left the front door open?”
✅ Correct sentence: “Who left the front door open?”
Explanation: The pronoun is performing the action (leaving the door open), so the subject form “who” is required.
Easy Memory Trick
Think of the letter M in whom. Both whroM and the word hiM, her, and theM end with the letter M. If the answer to your question would be him, her, or them, then you should use whom.
Mini Quiz
- Which sentence is best for a formal academic essay?
- A) Who wrote this historical document?
- B) By who was this historical document written?
- C) By whom was this historical document written?
- D) Whom wrote this historical document?
Answer: C
- Which choice is most natural in a casual conversation with a friend?
- A) By whom was the cake baked?
- B) Who baked the cake?
- C) Whom baked the cake?
- D) To whom did you speak?
Answer: B
- Fill in the blank: “The manager ________ approved the budget has left the company.”
- A) whom
- B) who
- C) by who
- D) by whom
Answer: B
- Which word pairs correctly with a preceding preposition in formal text?
- A) Who
- B) Whose
- C) Whom
- D) Which
Answer: C
Summary Table
| Situation | Correct Choice | Reason |
| Writing a formal research paper | By whom | Requires strict objective pronoun rules |
| Chatting with friends online | By who / Who… by | Casual speech favors “who” |
| Starting a passive voice question | By whom | Formal preposition placement |
| Rearranging words casually | Who… by | Places the preposition at the end |
Frequently Asked Questions (FAQs)
Is saying “by who” grammatically wrong?
In formal grammar, yes. Prepositions should be followed by objective pronouns like whom. However, in modern spoken English, it is widely accepted and common.
Can I end a sentence with “who”?
Yes, in casual English, restructuring the sentence to end with “who” (e.g., “Who was this designed by?”) is very natural and common.
When did people stop using whom?
The decline of “whom” began centuries ago as English simplified its case system. Today, it survives mostly in formal writing and fixed prepositional phrases.
How can I avoid using either phrase?
You can easily bypass the confusion by rewriting your sentence in the active voice. Instead of asking “By whom was this built?” you can ask “Who built this?”
Is “whom” completely dying out?
While it is rare in casual daily speech, “whom” remains a standard requirement in professional journalism, legal documents, and academic publishing.
Does Grammarly or spellcheck flag “by who”?
Advanced checkers will often flag “by who” in formal writing settings, suggesting “by whom” to maintain a professional tone.
What is the easiest way to test which one to use?
Try answering the question with a pronoun. If the answer is he, use who. If the answer is him, use whom.
Conclusion
Navigating the choice between these phrases ultimately comes down to understanding your audience and context. While formal rules dictate the use of the objective case, modern communication prioritizes clarity and natural flow.
Remember this simple rule: Use “by whom” for formal writing and professional documents, and use “by who” or active voice alternatives for everyday conversation.
I’m Sophie Caldwell, an education-focused writer who loves turning useful learning ideas into easy-to-understand guidance. I write about study routines, note-taking, exam preparation, productivity, and skills that can help students succeed academically. I want my content to feel supportive and practical, especially for students who sometimes feel overwhelmed by their studies. My aim is to make learning more organized, approachable, and enjoyable.
Books:
- Notes That Actually Work
- A Better Way to Study
Nathaniel Brooks