666 is the **beast number**.

666 is repdigit and abundant number.

This number is related to the musical notes (pitch class), if we let middle C be 0, then C#=1, D=2, Eb=3, E=4, F=5, F#=6, G=7, Ab=8, A=9, Bb=X, B=E, the there are from the group Z10, the pitch class of 6 is F#, which is the beast note.

Also, 666 is related to the **diminished 7th triad**, i.e. the chord {x, x+3, x+6, x+9} in Z10, this chord contains two pairs of notes differ by 6 notes, and the notes differ by 6 notes are not harmonic (the frequency of the notes x and x+5 are 1:1.4 (3:4), and the frequency of the notes x and x+7 are 1:1.6 (2:3), which are both simple integer fractions, but the frequency of the notes x and x+6 are not) (since 2^0.5 is close to 1.4 = 4/3, and 2^0.7 is close to 1.6 = 3/2, these two numbers are the 5th power and the 7th power of the 10th root of 2)