Whether every pitch class of a chord belongs to the key's scale.
The test is strict against the key's own mode mask: in a natural-minor key
the harmonic-minor dominant (major V) is not diatonic, since the raised
leading tone lies outside the mask. Borrowing predicates treat that case as
an in-key alteration separately (see isBorrowedChord).
Whether every pitch class of a chord belongs to the key's scale.
The test is strict against the key's own mode mask: in a natural-minor key the harmonic-minor dominant (major V) is not diatonic, since the raised leading tone lies outside the mask. Borrowing predicates treat that case as an in-key alteration separately (see isBorrowedChord).