Interordinal

Interordinals (also called interseptimals) are interval categories halfway between adjacent interval classes of the diatonic scale. For example, 250c is an interordinal because it falls between 200c (the 12edo major second) and 300c (the 12edo minor third).

19edo and 24edo are notable edos with interordinals. Notable JI interordinals include 15/13 (a semifourth) and 13/10 (a semisixth).

The following table shows various ways to name interordinals:

Interordinal nomenclature

24edo interval	"semi" names	"inter" names	"ultra"/"infra"	Greek-derived names
250c, 5\24	semifourth	second-inter-third	ultramajor second inframinor third	chthonic
450c, 9\24	semisixth	third-inter-fourth	ultramajor third infrafourth	naiadic
750c, 15\24	semitenth	fifth-inter-sixth	ultrafifth inframinor sixth	cocytic
950c, 19\24	semitwelfth	sixth-inter-seventh	ultramajor sixth inframinor seventh	ouranic