18edt

18edt means the division of the tritave into 18 equal parts of size 105.664 cents each. It has a decent 7 and an excellent 13, but a 5 which is 39 cents flat; if octaves were added and it was a sixth, it would count as a neutral sixth. The corresponding 5/3 is 845 cents, which is a neutral sixth between 8/5 and 5/3, which is really more of a 13/8, though this is allegedly a no-twos tuning. With octaves added, it also has a minor third and a major tenth which are both excellent as well as a minor thirteenth and major seventeenth which are still decent even though it skips actual octaves (in fact it is the non-octave semitone scale of 34edo). On the 3.7.13 subgroup it tempers out 351/343 and 2197/2187.

As the double of 9edt, it is the analog of 14edo insofar as it has a doubled harmonic chain. However, it, like 8edt, is not schismatic because 3:5:7 is a redundant chord. As a multiple of 9edt, it is the widest variety of 'White-Extraterrestrial-Tree' temperament.

step	cents	hekts	approximated interval
0	0.000		1/1
1	105.664	72.222	16/15
2	211.328	144.444	9/8
3	316.993	216.667	6/5
4	422.657	288.889	9/7
5	528.321	361.111	27/20
6	633.985	433.333	13/9
7	739.649	505.556	17/13
8	845.313	577.778	5/3
9	950.978	650	19/11
10	1056.642	722.222	9/5
11	1162.306	794.444	49/25
12	1267.97	866.667	27/13
13	1373.634	938.889	20/9
14	1479.298	1011.111	7/3
15	1584.963	1083.333	5/2
16	1690.627	1155.556	8/3
17	1806.291	1227.778	45/16
18	1901.955	1300	3/1