Diminished tuning is a system of choosing the reeds for a diatonic wind instrument (such as a harmonica or accordion) in which the blow notes repeat a sequence of C E♭ F♯ A and draw notes follow a repeating sequence of D F G♯ B (perhaps shifted to begin with E♭ and F, with F♯ and G♯, or with A and B).
For example:
{ | class="wikitable" style="text-align:center" ! hole || 1 || 2 || 3 || 4 || 5 || 6 || 7 || 8 || 9 || 10 || 11 || 12 |- ! blow note || C || E || F || A || C || E || F || A || C || E || F || A |- ! draw note || D || F || G♯ || B || D || F || G♯ || B || D || F || G♯ || B |}