A MIDI sound file that contains MIDI messages. MIDI files used in DOS and Windows have a .MID extension. A variation of this format is the RIFF MIDI file, which uses the .RMI extension. The format of a Standard MIDI File (SMF) contains a header "chunk" at the beginning that defines the type, followed by one or more track chunks. Type 0 files store all tracks in one track chunk. Type 1 files use a separate chunk for each track, with the first chunk storing the tempo. Type 0 files use less memory and run faster than type 1. Thus, original MIDI music is maintained in type 1 format and frequently distributed in type 0. MIDI files distributed for editing are usually in type 1 format, since it is difficult to convert from type 0 to type 1 using a MIDI sequencer. A less-widely used type 2 file can contain several type 0 files.