What "Fronting" a Preposition Means

In everyday English, when a relative clause contains a preposition, most people stick the preposition at the end:

That's the hotel that we stayed at.

There's another option: move the preposition to the front of the relative clause, right before the relative pronoun. When you do this, you must use whom (for people) or which (for things) — never that, and never leave the pronoun out:

That's the hotel at which we stayed.

This is called a fronted preposition (sometimes "pied-piping," if you like linguistic jargon). The meaning doesn't change, but the sentence sounds more formal and more precise. You'll meet it constantly in academic writing, official reports, and literary prose.

How the Pattern Works — Two Worked Examples

Example 1 — thing (which)

Imagine you want to combine these two ideas: The bridge was built in 1890. We walked across the bridge.

✅ The bridge across which we walked was built in 1890. ❌ The bridge across that we walked was built in 1890.

The second sentence is wrong because that cannot follow a preposition. Once you pull the preposition to the front, you need which.

Example 2 — person (whom)

Combine: The colleague helped me a lot. I was working with the colleague.

✅ The colleague with whom I was working helped me a lot. ❌ The colleague with who I was working helped me a lot.

After a fronted preposition, the correct form for people is whom, not who. In casual speech people bend this rule, but in writing — and certainly in exams — whom is expected.

When to Front and When Not To

Fronting the preposition is optional in most sentences. You can always leave it at the end instead:

The colleague who/that I was working with helped me a lot.

Both versions are grammatically correct. The fronted version is simply more formal.

However, there are contexts where fronting sounds strange and you should avoid it:

• Phrasal verbs. If the preposition is really a particle that belongs tightly to the verb, don't front it. Nobody writes "The noise up with which I will not put" — that's a famous joke, not real English. Keep phrasal-verb particles at the end: the noise that I won't put up with.
• Informal registers. In a text message or chatty email, "the guy to whom I was talking" can sound oddly stiff. Match your register.

On the other hand, fronting is almost expected when the sentence already has a formal tone — contracts, research papers, or any writing where clarity and precision matter more than warmth.

The Mistake B1–B2 Learners Make Most Often

The single most common error is using that after a fronted preposition:

❌ The project in that she was involved required extra funding. ✅ The project in which she was involved required extra funding.

This happens because learners know that works as a general-purpose relative pronoun, and it does — but only when the preposition stays at the end (the project that she was involved in). The moment you move the preposition forward, that is no longer allowed. Swap it for which (things) or whom (people), and the problem disappears.

A related slip is doubling the preposition — fronting it and accidentally leaving a copy at the end:

❌ The city in which we grew up in was tiny. ✅ The city in which we grew up was tiny.

If you've moved the preposition to the front, it's already doing its job. Delete the leftover one at the end.

Keep these two checks in mind — no that after a preposition, and no doubled preposition — and fronted relative clauses become a reliable tool for making your writing sound polished and precise.