»prompt side

According to the Theatre Protective Union Local No. 1, the "prompt side" comes from opera, and means "stage right": "Our backstage tour began on the prompt side of the stage (many of the Radio City stagehands use the opera term prompt side for stage right)". I read it initially in a P G Wodehouse story describing the weekend-morning mob at Marble Arch, replete with Communists, sham-artists, and run-of-the-mill lunatics.