"What's My Line?" was a guessing game in which the four panelists attempted to determine the occupation (i.e., "line work") of a guest. In the case of the famous mystery guest each week, the panel sought to determine the identity of the celebrity. Panelists were required to probe by asking only yes-no questions. A typical episode featured two standard rounds (sometimes a third, and very rarely a rushed fourth) plus one mystery guest round. On the occasions on which there were two mystery guests, the first would usually appear as the first contestant.