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Classification of musical scale or key in ancient Greek music theory
In the musical system of ancient Greece, genus (Greek: γένος [genos], pl. γένη [genē], Latin: genus, pl. genera "type, kind") is a term used to describe certain classes of intonations of the two movable notes within a tetrachord. The tetrachordal system was inherited by the Latin medieval theory of scales and by the modal theory of Byzantine music; it may have been one source of the later theory of the jins of Arabic music. In addition, Aristoxenus (in his fragmentary treatise on rhythm) calls some patterns of rhythm "genera".

According to the system of Aristoxenus and his followers—Cleonides, Bacchius, Gaudentius, Alypius, Bryennius, and Aristides Quintilianus—the paradigmatic tetrachord was bounded by the fixed tones hypate and mese, which are a perfect fourth apart and do not vary from one genus to another. Between these are two movable notes, called parhypate and lichanos. The upper tone, lichanos, can vary over the range of a whole tone, whereas the lower note, parhypate, is restricted to the span of a quarter tone. However, their variation in position must always be proportional. This interval between the fixed hypate and movable parhypate cannot ever be larger than the interval between the two movable tones. When the composite of the two smaller intervals is less than the remaining (incomposite) interval, the three note group is called pyknon (meaning "compressed"). The positioning of these two notes defined three genera: the diatonic, chromatic (also called chroma, "colour"), and enharmonic (also called ἁρμονία [harmonia]). The first two of these were subject to further variation, called shades—χρόαι (chroai)—or species—εἶδη (eidē). For Aristoxenus himself, these shades were dynamic: that is, they were not fixed in an ordered scale, and the shades were flexible along a continuum within certain limits. Instead, they described characteristic functional progressions of intervals, which he called "roads" (ὁδοί), possessing different ascending and descending patterns while nevertheless remaining recognisable. For his successors, however, the genera became fixed intervallic successions, and their shades became precisely defined subcategories. Furthermore, in sharp contrast to the Pythagoreans, Aristoxenos deliberately avoids numerical ratios. Instead, he defines a whole tone as the difference between a perfect fifth and a perfect fourth, and then divides that tone into semitones, third tones, and quarter tones, to correspond to the diatonic, chromatic, and enharmonic genera, respectively.

Aristoxenus describes the diatonic genus (διατονικὸν γένος) as the oldest and most natural of the genera. It is the division of the tetrachord from which the modern diatonic scale evolved. The distinguishing characteristic of the diatonic genus is that its largest interval is about the size of a major second. The other two intervals vary according to the tunings of the various shades.

The English word diatonic is ultimately from the διατονικός, itself from διάτονος, of disputed etymology. Most plausibly, it refers to the intervals being "stretched out" in that tuning, in contrast to the other two tunings, whose lower two intervals were referred to as πυκνόν, from πυκνός. This takes τόνος, to mean "interval of a tone"; see Liddell and Scott's Greek Lexicon and Barsky (second interpretation), below. Alternatively, it could mean (as OED claims) "through the tones", interpreting διά as "through". See also Barsky: "There are two possible ways of translating the Greek term 'diatonic': (1) 'running through tones', i. e. through the whole tones; or (2) a 'tensed' tetrachord filled up with the widest intervals". The second interpretation would be justified by consideration of the pitches in the diatonic tetrachord, which are more equally distributed ("stretched out") than in the chromatic and enharmonic tetrachords, and are also the result of tighter stretching of the two variable strings. It is perhaps also sounder on linguistic morphological grounds. Compare diameter as "across/width distance". A completely separate explanation of the origins of the term diatonic appeals to the generation of the diatonic scale from "two tones": "Because the musical scale is based entirely on octaves and fifths, that is, two notes, it is called the 'diatonic scale' ". But this ignores the fact that it is the element di that means "two", not the element dia , which has "through" among its meanings (see Liddell and Scott). There is a Greek term δίτονος, which is applied to an interval equivalent to two tones. It yields the English words ditone and ditonic (see Pythagorean comma), but it is quite distinct from διάτονος. The Byzantine theorist George Pachymeres consider the term derived from διατείνω, meaning "to stretch to the end", because " the voice is most stretched by it"
Yet another derivation assumes the sense "through the tones" for διάτονος, but interprets tone as meaning individual note of the scale: "The word diatonic means 'through the tones' (i. e., through the tones of the key)" (Gehrkens, 1914, see ; see also the Prout citation, at the same location). This is not in accord with any accepted Greek meaning, and in Greek theory it would fail to exclude the other tetrachords. The fact that τόνος itself has at least four distinct meanings in Greek theory of music contributes to the uncertainty of the exact meaning and derivation of διατονικός, even among ancient writers: τόνος may refer to a pitch, an interval, a "key" or register of the voice, or a mode.

In Byzantine music most of the modes of the octoechos are based on the diatonic genus, apart from the second mode (both authentic and plagal) which is based on the chromatic genus. Byzantine music theory distinguishes between two tunings of the diatonic genus, the so called "hard diatonic" on which the third mode and two of the grave modes are based, and the "soft diatonic" on which the first mode (both authentic and plagal) and the fourth mode (both authentic and plagal) are based. The hard tuning of the diatonic genus in Byzantine music may also be referred to as the enharmonic genus; an unfortunate name that persisted, since it can be confused with the ancient enharmonic genus.

The number and nature of the shades of the chromatic genus vary amongst the Greek theorists. The major division is between the Aristoxenians and the Pythagoreans. Aristoxenus and Cleonides agree there are three, called soft, hemiolic, and tonic. Ptolemy, representing a Pythagorean view, held that there are five.

In Byzantine music the chromatic genus is the genus on which the second mode and second plagal mode are based. The "extra" mode nenano is also based on this genus.

Aristoxenus describes the enharmonic genus (; ) as the "highest and most difficult for the senses". Historically it has been the most mysterious and controversial of the three genera. Its characteristic interval is a ditone (or major third in modern terminology), leaving the pyknon to be divided by two intervals smaller than a semitone called dieses (approximately quarter tones, though they could be calculated in a variety of ways). Because it is not easily represented by Pythagorean tuning or meantone temperament, there was much fascination with it in the Renaissance. In the modern tuning system of twelve tone equal temperament, enharmonic refers to tones that are identical, but spelled differently. In other tuning systems, enharmonic notes, such as C and D, may be close but not identical, differing by a comma (an interval smaller than a semitone, like a diesis).

The principal theorist of rhythmic genera was Aristides Quintilianus, who considered there to be three: equal (dactylic or anapestic), sesquialteran (paeonic), and duple (iambic and trochaic), though he also admitted that some authorities added a fourth genus, sesquitertian.