An offer of proof—also known as an informal bill of exception—is a procedure available during trial for the presentation of evidence the trial court has excluded. An offer of proof is made outside of the jury’s presence and often after the judge has sustained an objection to the admissibility of the evidence.

A bill of exception—also known as a formal bill of exception—is a procedure that allows a party to complain on appeal about evidence that would not otherwise appear in the trial record (because there was no offer of proof made at trial) by detailing the evidence (testimony, etc.) that would have been offered if the trial court had allowed it.

In California, an offer of proof is a procedural mechanism used during a trial when the court excludes evidence. This process allows the party attempting to introduce the evidence to make a record of what the evidence would have shown, which is done outside the presence of the jury. The purpose of an offer of proof is to preserve the excluded evidence for appellate review, by demonstrating how it was relevant and material to the case. On the other hand, a formal bill of exception, which is less commonly used under modern rules of appellate procedure, serves a similar purpose. It is a written statement by the judge detailing the evidence that was not allowed, which is prepared after a party's request when there was no offer of proof made during the trial. This document becomes part of the trial record and can be used to argue that the trial court erred in excluding the evidence. Both procedures are designed to ensure that the appellate court is aware of any potentially prejudicial errors regarding the admission or exclusion of evidence during the trial.