# 666edo

← 665edo | 666edo | 667edo → |

^{2}× 37**666 equal divisions of the octave** (abbreviated **666edo** or **666ed2**), also called **666-tone equal temperament** (**666tet**) or **666 equal temperament** (**666et**) when viewed under a regular temperament perspective, is the tuning system that divides the octave into 666 equal parts of about 1.8 ¢ each. Each step represents a frequency ratio of 2^{1/666}, or the 666th root of 2.

## Theory

666edo is enfactored in the 17-limit, with the same mapping as 333edo, but the 2.9.15.21.11.17.19 subgroup is great for 666edo. Alternatively, it can be used with 2.9.15.7.11.19.23. 666edo provides good direct approximations for: 15/11, 16/11, 16/15, 13/12, 13/10, 22/15, 23/14. 666edo also has a strong approximation for 11/8 derived from 37edo and for 7/6 derived from 9edo, but on the 2.7/6.11 subgroup it is enfactored, with the same tuning again as 333edo, tempering out the 37-11-comma and the septimal ennealimma. The 666c val, tempers out 2401/2400, 4375/4374, and 9801/9800 in the 11 limit.

666edo is also used by Eliora to approximate the "Factor 9 grid", or the just intonation esoteric scale deconstructed and debunked by Adam Neely. Now, it may be worth noting that the tuning system which truly has an excellent approximation of the Factor 9 grid is 666ed15/14, approximately equivalent to 6691edo. However, this fact was not spotted by Eliora until after first music was composed in 666edo due to the temperament finder layout making it not immediately obvious what is the interval of equivalence, so the Factor 9 grid representation by 666edo still remains notable given that it is a scale for some of the first music composed in this edo.

### Odd harmonics

Harmonic | 3 | 5 | 7 | 9 | 11 | 13 | 15 | 17 | 19 | 21 | 23 | |
---|---|---|---|---|---|---|---|---|---|---|---|---|

Error | Absolute (¢) | +0.748 | -0.728 | +0.543 | -0.306 | +0.033 | -0.888 | +0.020 | -0.451 | -0.216 | -0.511 | +0.554 |

Relative (%) | +41.5 | -40.4 | +30.2 | -17.0 | +1.9 | -49.3 | +1.1 | -25.0 | -12.0 | -28.3 | +30.8 | |

Steps (reduced) |
1056 (390) |
1546 (214) |
1870 (538) |
2111 (113) |
2304 (306) |
2464 (466) |
2602 (604) |
2722 (58) |
2829 (165) |
2925 (261) |
3013 (349) |

### Subsets and supersets

Since 666 factors into 2 × 3^{2} × 37, 666edo has subset edos 2, 3, 6, 9, 18, 37, 74, 111, 222, 333, of which 111edo is a notable system due to its accuracy relative to its size.

## Regular temperament properties

### Rank-2 temperaments

Periods per 8ve |
Generator* | Cents* | Associated Ratio* |
Temperaments |
---|---|---|---|---|

1 | 175\666 | 315.31 | 6/5 | Parakleismic (666b) |

9 | 35\666 | 63.063 | 336/323 | Enneasoteric |

9 | 175\666 (27\666) |
315.315 (48.648) |
6/5 (36/35) |
Ennealimmal (666c) |

18 | 138\666 (27\666) |
248.648 (48.648) |
15/13 (99/98) |
Hemiennealimmal (666cf) |

* octave-reduced form, reduced to the first half-octave, and minimal form in parentheses if it is distinct

## Scales

- Factor9Grid[14]: 39 38 36 35 66 62 59 55 52 49 46 44 42 41

## Music

*Timeline*(2022) – classical piano*Church of Original Sin*(2022) – as part of the symphonic metal project Mercury Amalgam