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 Washington State, an offer of proof is a procedural tool used during a trial when the court excludes evidence. This process allows the attorney to create a record of what the evidence would have shown, which is essential for appellate review. The offer of proof is typically made outside the presence of the jury, following a judge's ruling that sustains an objection to the evidence's admissibility. On the other hand, a bill of exception, or formal bill of exception, is a mechanism used to preserve for appeal the details of evidence that was not admitted and for which no offer of proof was made during the trial. This procedure ensures that the appellate court can review the nature of the excluded evidence even though it was not part of the trial record. Both processes are critical for ensuring that a party can challenge evidentiary rulings on appeal and are governed by the Washington Rules of Evidence and relevant case law.