4. Trials: The Rule
There are thousands of rules of procedure and evidence that affect the conduct of cases in Texas courts. Of these rules, only one of them is referred to as The Rule.
Before the beginning of evidence in most cases, the judge will ask, “Does either side wish to invoke the rule?” By this question, the judge is asking whether the attorneys wish to have the witnesses excluded from the courtroom so they cannot hear the testimony as it is being given. If either side desires that the rule be invoked, the judge will swear all the witnesses present and instruct them that they are to remain outside the courtroom during the testimony and that they are not to discuss the case with anyone except the lawyers for both sides.
Of course, the parties to the litigation (you and your spouse) are excluded from the rule and are allowed to remain in the courtroom. On other occasions, the court may except other persons from the rule, such as expert witnesses, who are needed by the attorneys to aid in litigation. Lawyers sometimes use the rule to exclude persons from the courtroom whom they do not want present and whom they never intend to call as witnesses.
If the rule is broken, it is very likely that the witness involved will not be allowed to testify.
Robin M. Green, Divorce: When It’s the Only Answer (The Ordinary Mortals Guide, Inc., 2005), Chapter 12, pp. 168-169.