Messiaen used modes he called "modes of limited transposition". They are distinguished as groups of notes that can only be transposed by a semitone a limited number of times. For example, the whole-tone scale (Messiaen's Mode 1) only exists in two transpositions: namely C–D–E–F–G–A and D–E–F–G–A–B. Messiaen abstracted these modes from the harmony of his improvisations and early works. Music written using the modes avoids conventional diatonic harmonic progressions, since for example Messiaen's Mode 2 (identical to the "octatonic scale" used also by other composers) permits precisely the dominant seventh chords whose tonic the mode does not contain.