At a trial in the American court system, attorneys often ask to "invoke the rule." This is a reference to Rule of Evidence 615 (individual states may designate this rule with a different number), which allows for the sequestering of witnesses during trial. What this means is that witnesses to be called during the trial are to be excluded from the courtroom until they are to testify. It is an attempt to keep the testimony of witnesses pure and uninfluenced by the testimony of other witnesses.