Prosecution history

Everything you argued to get the patent granted is public, permanent, and available to the person you later sue.

In simple words

The complete record of correspondence between the applicant and the Patent Office - objections, replies, amendments and arguments - which anyone can inspect after publication.

Think of it as

A transcript of every concession you made.

Why it matters

If you told the examiner your invention was distinguished from the prior art because it required feature X, it is difficult to argue later that a competitor without feature X infringes. What you say to get past an objection narrows what you can assert.

A simple example

To overcome an obviousness objection you argue that the essential difference is a two-stage filter. A competitor uses a single-stage filter. Your own file makes their non-infringement argument for them.

Common mistake

Writing FER replies as though nobody will ever read them again. They are the first thing an opponent's counsel reads.

Where it comes from

Patents Act 1970, section 153 (inspection of documents); Patents Rules 2003, rule 27