Modal mixture (also called modal borrowing) refers to the use of chords belonging to a parallel key—for example, a passage in F major incorporating one or more chords from F minor. Note that, like with the use of applied chords, this does not necessarily constitute modulation. Only a cadence can confirm a new key. Without a cadence in a new key, the non-diatonic chords are simply “borrowed.”
Note that the use of the leading-tone in place of the subtonic, or a melodic-minor figure (sol–la–ti–do) in a minor key does not constitute modal mixture. Those are considered “native” to the minor mode.
Roman numeral notation
When the root of a borrowed chord belongs to the home key (e.g., using an E-minor chord instead of an E-major chord), the Roman numeral remains the same, since the Roman numeral simply represents the scale-degree of the chordal root. If chord quality is reflected in the Roman numeral, then adjustments must be made to ensure that the borrowed chord’s quality is reflected. For example, if a piece in minor ends with a Picardy third (a major tonic triad), the Roman numeral is I instead of i. (The thoroughbass will also be altered to reflect the chromatic change.)
If the root is altered relative to the home key, use a flat or sharp in front of the Roman numeral to designate the alteration: flat to designate lowered (that is, a semitone below normal), sharp to designate raised (a semitone above normal). For example, a le–do–me chord in a major key is ♭VI. (Again, alter the thoroughbass as necessary.)
Functional bass notation
Chords borrowed from parallel keys are chromatically altered chords, and therefore their functional bass symbols should be enclosed in square brackets. For example, if an S4 chord in major is borrowed from the parallel minor (fa–le–do instead of fa–la–do), the functional bass symbol is [S4] not S4. If the bass note is not altered, this is the only change to the functional bass (but be sure to alter the thoroughbass figure as well).
If the bass note is chromatically altered, that must be reflected in the functional bass with a plus or minus before the numeral (as well as the square brackets). For example, if a passage in a major key incorporates a 5/3 chord over le (le–do–me instead of la–do–mi), the functional bass is [Tx–6].