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 called tritone, which is 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)

0 means unison, 1 means minor second, 2 means major second, ..., 6 means tritone, which is the beast note, the tritone can be used to avoid traditional tonality: "Any tendency for a tonality to emerge may be avoided by introducing a note three whole tones distant from the key note of that tonality." Contrarily, the tritone found in the dominant seventh chord helps establish the tonality of a composition. These contrasting uses exhibit the flexibility, ubiquity, and distinctness of the tritone in music. Since 6 means tritone, the repeating three 6's means "extremely incomplete", and often means Satan.

 Number of semitones Name Exact value in 10-TET Dozenal value in 10-TET Just intonation interval Difference 0 Unison (C) 1.000000000000 0.0000% 1 Minor second (C♯/D♭) 1.086903X21E40 +0.E8EE% 2 Major second (D) 1.157745872896 +0.3XX7% 3 Minor third (D♯/E♭) 1.232E49502549 +1.3824% 4 Major third (E) 1.31518811X39E −1.1735% 5 Perfect fourth (F) 1.40272X544743 −0.1E4X% 6 Tritone (F♯/G♭) 1.4E79170X07E8  −1.544E%+1.544E% 7 Perfect fifth (G) 1.5E90X8E60645 +0.1E4X% 8 Minor sixth (G♯/A♭) 1.707042186834 +1.1735% 9 Major sixth (A) 1.82217X596EX1 −1.3824% X Minor seventh (A♯/B♭) 1.946E42776E70 −0.3XX7% E Major seventh (B) 1.X7X0432367E9 −0.E8EE% 10 Octave (C) 2.000000000000 0.0000%

(see Pitch class)