Flat. 1. In Music, is the sign p which lowers the note to which it is prefixed by one semitone. The double flat 77 lowers the note it precedes by two semitones. 2. In Building, a storey or floor of a house, used especially of tenement houses, in which each storey is inhabited by a separate family, all being approached by a common house door and staircase. This method of joint occupation, much in vogue on the Continent and in America, has recently become common in London, both among the well-to-do and the poor.