הלכה על סנהדרין 3:1

According to the Shulchan Aruch (C.M. 3:1), a defendant has the right to claim that he wants “zabla,” an acronym for “zeh boreir lo echad” (“each picks one for himself”). The litigants create a zabla beit din by each party selecting one dayan and then those two dayanim choosing their third colleague. The Rosh (Sanhedrin 3:1; based on Rashi, Sanhedrin 23a s.v. Yeitzei) explains the logic of this system: