A) An indication of all sources that are contributing to a switcher’s final output at any given time. b) A light which lights up to indicate that the associated push-button has been selected or to indicate that the associated input to the switcher is on-air. c) A relay closure to activate a remotely situated lamp, i.e., on top of a camera, to warn the production crew which camera is on-air. Most monitors have tally lights and common practise is to connect them to the switcher tally output so that the director can see which source is on-air.