A musical quotation within a film makes direct reference to the quoted music as it exists outside of that film. Among the most common functions of pre-existing music in film are the establishment of historical and geographical setting. In establishing a past temporal setting in particular, films have most prominently employed pre-existing popular music. Pre-existing music is, literally, real music that refers to its own existence outside of the film. Its deployment as source music can therefore be a particularly effective means of authentically furnishing real-world settings, much like the sight of a Starbucks café or Ford car. Source music can be on-screen or off-screen, those labels referring to whether the music's implied source is within or outside of the film's visual frame. If the music seems to represent the sonic environment in which the characters exist – more as music that they would hear than music they could hear – then labelling it as diegetic seems fair.