Messiaen was fascinated by scales that only had a few (usually two or three) transpositions. For example: the whole tone scale, which rises by a whole tone each time, has two transpositions. It can start, for example, on a C, so that the notes are C, D, E, F#, G#, A#, C. It can start on a C# so that the notes are C#, D#, F, G, A, B, C#. Transposing this up another semitone, starting on D, would give D, E, F#, G#, A#, C, D which is exactly the same combination of notes as the first one (the starting note is not important). The whole-tone scale was used by many composers including Glinka, Liszt and, especially, Debussy. Messiaen called it the "first mode" of transposition.