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81/80
|-4 4 -1>
21.506290 cents

The syntonic or Didymus comma (frequency ratio 81/80) is the smallest superparticular interval which belongs to the 5-limit. Like 16/15, 625/624, 2401/2400 and 4096/4095 it has a fourth power as a numerator. Fourth powers are squares, and any comma with a square numerator is the ratio between two larger successive superparticular intervals; it is in fact the difference between 10/9 and 9/8, the product of which is the just major third, 5/4. That the numerator is a fourth power entails that the larger of these two intervals itself has a square numerator; 9/8 is the interval between the successive superparticulars 4/3 and 3/2. 105edo tempers it out, while 3edo does not.

Tempering out 81/80 gives a tuning for the whole tone which is intermediate between 10/9 and 9/8, and leads to meantone temperament.

Youtube video of "Five senses of 81/80", demonstratory video by Jacob Barton.

According to this interview, Monroe Golden's Incongruity uses just-intonation chord progressions that exploit this comma.

Relations to other Superparticular Ratios

Superparticular ratios, like 81/80, can be expressed as products or quotients of other superparticular ratios. Following is a list of such representations r1 * r2 or r2 / r1 of 81/80, where r1 and r2 are other superparticular ratios.
Names in brackets refer to 7-limit meantone extensions, or 11-limit rank three temperaments from the Didymus family that temper out the respective ratios as commas.
Limit
r1 * r2
r2 / r1
5
-
9/8 * 9/10
7
126/125 * 225/224 (septimal meantone)
21/20 * 27/28 (sharptone), 36/35 * 63/64 (dominant)
11
99/98 * 441/440 (euterpe), 121/120 * 243/242 (urania)
33/32 * 54/55 (thalia), 45/44 * 99/100 (calliope)
13
91/90 * 729/728, 105/104 * 351/350
27/26 * 39/40, 65/64 * 324/325, 66/65 * 351/352, 78/77 * 2079/2080
17
85/84 * 1701/1700
51/50 * 135/136
19
96/95 * 513/512, 153/152 * 171/170
57/56 * 189/190, 76/75 * 1215/1216, 77/76 * 1539/1540
23
161/160 * 162/161
69/68 * 459/460
29
117/116 * 261/260
-
31
93/92 * 621/620
63/62 * 279/280
37
111/110 * 297/296
75/74 * 999/1000
41
82/81 * 6561/6560
41/40 * 81/82
43
86/85 * 1377/1376, 87/86 * 1161/1160, 129/128 * 216/215
-
47
141/140 * 189/188
-
53
-
54/53 * 159/160
59
-
-
61
-
61/60 * 243/244
67
135/134 * 201/200
-
71
-
71/70 * 567/568, 72/71 * 639/640
73
-
73/72 * 729/730
79
-
79/78 * 3159/3160, 80/79 * 6399/6400
83
83/82 * 3321/3320, 84/83 * 2241/2240
-
89
89/88 * 891/890, 90/89 * 801/800
-
97
97/96 * 486/485
-
101
101/100 * 405/404
-
103
-
-
107
108/107 * 321/320
-

External Links

http://en.wikipedia.org/wiki/Syntonic_comma