It is the number of half-steps in a tritone as well as the number of notes in a whole-tone scale.  
This is the number of pitch-classes in Pierre Boulez's "Sacher" chord.  In tonal music, one common pre-dominant treatment is the augmented this chord, which depending on its spelling can come in Italian, French, or German varieties.  This is the number of notes in the augmented scale, and in a diatonic scale this scale degree, which is flattened in the harmonic minor scale but not the ascending melodic minor, is known as the submediant, as well as la in solfege. It is the number of half-steps in a tritone as well as the number of notes in a whole-tone scale.  For 10 points, give this number of Mahler's Tragic Symphony which also refers to a group comprised of Georges Auric, Darius Milhaud, and four other Frenchmen.
This is the number of pitch-classes in Pierre Boulez's "Sacher" chord.  
{six} (be {generous} and accept "{sixth}" as {well} as a {late} "{Les Six}")