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 Montana, an offer of proof is a procedural tool used during a trial to create a record of evidence that the court has deemed inadmissible. This is done outside the presence of the jury, typically after an objection to the evidence has been sustained by the judge. The purpose of an offer of proof is to preserve the excluded evidence for appellate review, by demonstrating what the evidence would have shown and why it is relevant and material to the case. On the other hand, a bill of exception, which is less commonly used in modern practice due to changes in appellate procedures, is a formal written statement by a trial judge or an attorney detailing the evidence or objections that were not included in the trial record. This procedure is designed to ensure that an appellate court can review decisions about evidence that was not part of the offer of proof at trial. Both mechanisms serve to protect a party's right to appeal by ensuring that the appellate court is aware of all evidence and objections, even those not reflected in the main trial record.