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 Colorado, 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 crucial for appellate review. The offer of proof is typically made outside the jury's presence, ensuring that the jury is not influenced by evidence deemed inadmissible. This is done after an objection to the evidence has been sustained by the judge. On the other hand, a bill of exception, or formal bill of exception, is used to preserve a record for appellate review when no offer of proof was made during the trial. It details the evidence that would have been presented if the court had not excluded it. This procedure ensures that the appellate court can consider whether the trial court's exclusion of evidence was erroneous and whether it affected the trial's outcome. Both mechanisms are essential for preserving issues for appeal when a party believes that the trial court made a legal error by excluding evidence.