The Lightner double is a conventional double in bridge, used to direct the opening lead against slam contracts.
The Lightner double is a call made by the partner of the player who will make the opening lead.
The opening lead is often crucial to the play of the hand, and the right opening lead is often the only chance for the defenders to defeat the contract.
The most common interpretation is to lead the first suit (other than trumps) bid by the opponents.
"Skid" Simon called it "one of the most brilliant contributions to Contract Bridge yet made".