Dominant seventh chord

English Wikipedia has an article on:

Dominant seventh chord

A dominant seventh chord is a tetrad comprising a root, a major third, a fifth, and a minor seventh. The name of the chord derives from the dominant[1] scale degree, which is the only degree of a diatonic scale on which it is found. However, in many musical genres, “dominant seventh chord” informally refers to any chord with this general structure, regardless of where it appears in the overall scale.

JI Tunings

In the 3-limit:

• 72:81:108:128 (1⁄1–81⁄64–3⁄2–16⁄9), the Pythagorean dominant seventh chord, is found on the dominant scale degree (3⁄2 or V) of the Pythagorean diatonic scale.

In the 5-limit:

• 36:45:54:64 (1⁄1–5⁄4–3⁄2–16⁄9), the the Ptolemaic dominant seventh chord, is found on the dominant scale degree (3⁄2 or V) of Zarlino, perhaps the most common 5-limit diatonic.

• 20:25:30:36 (1⁄1–5⁄4–3⁄2–9⁄5), the major-minor seventh chord, combines a major third with the consonant seventh that would be found in a ptolemaic minor seventh chord built on the same root. It is found on the minor chords rooted at 5⁄4 (III) and 15⁄8 (VI) in the duodene.

• 108:135:160:192 (1⁄1–5⁄4–40⁄27–16⁄9) is found on the dominant scale degree if the second degree of the diatonic scale is tuned a comma lower than in Zarlino (10⁄9 instead of 9⁄8).

• 128:160:192:225 (1⁄1–5⁄4–3⁄2–225⁄128), the Neapolitan chord, is found on the chords rooted at 16⁄15 (♭II) and 8⁄5 (♭VI) in the duodene. (Although 225⁄128 is often considered an augmented sixth rather than a minor seventh, in septimal meantone it is tuned identically to the harmonic seventh chord.)

In the 7-limit:

• 4:5:6:7 (1⁄1–5⁄4–3⁄2–7⁄4), the harmonic seventh chord is a consonant chord in the 7-limit, often used as a tuning target in barbershop music[2].