18edo

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18edo, or 18 equal divisions of the octave, is the equal tuning featuring steps of (1200/18) ~= 66.7 cents, 18 of which stack to the octave 2/1.

With the sharp fifth 733.3c and the flat fifth 666.7c almost equally detuned from the just fifth, 18edo is often considered the quintessential straddle-3 edo and the straddle-3 version of 12edo. It does not approximate low harmonics well (except 9 and debatably 5); it is also straddle-7, 13, 17, and 19.

Scales

• Straddle-3 diatonic (5L1m1s), 3331332 or 3332331, constructed by an alternating stack of flat and sharp fifths
• Oneirotonic (5L3s), 33133131 (compressed 17edo diatonic)
• Smitonic (4L3s), 3323232 (stretched 19edo diatonic)
• Taric (8L2s), 2222122221 and the altered MOS pentachordal taric, 2221222221
• Hexawood (6L6s) is a "straddle-3 chromatic scale", constructed by an alternating stack of flat and sharp fifths

DR theory

18edo has the following approximate DR chords:

• [0 8 14]\18 (approx. +1+1)
• [0 6 11 15]\18 (approx. +1+1+1)
• [0 5 9 16]\18 (approx. +1+1+2)
• [0 3 8]\18 (approx. +1+2)
• [0 5 13]\18 (approx. +1+2)
• [0 5 11]\18 (approx. +2+3)
• [0 6 13]\18 (very accurately +2+3)
• [0 4 8 11]\18 (approx. +1+?+1)
• [0 3 11 13]\18 (approx. +1+?+1)
• [0 7 10 15]\18 (approx. +1+?+1)

etc.