Generalised notation for polyhedral dice in role-playing games. Formally read, ndm means 'generate n random numbers each from 1 to m, and sum them'. n and m are used because x and y are traditionally used for continuous variables, and n and m (in that order) for discrete variables. Common examples of this are the 3d6 ability score roll in Advanced Dungeons and Dragons and the various sanity loss rolls in Call of Cthulhu. Recent practice omits n when it is equal to 1.