Although in minor keys we usually use the notes from the harmonic minor scale to build the subdominant chord, it’s also possible to build a different chord, using the notes from the melodic minor scale.
In the ascending melodic minor scale, the 6th of the scale is raised by a semitone, as well as the 7th. Using this note creates a major subdominant chord, instead of the more typical minor subdominant.
The major subdominant triad in A minor contains the notes D, F# and A.
Because the chord is major, we use capital letters to describe the chord.
The major subdominant chord is rare compared the minor version. Like minor chord v, it’s sometimes used to harmonise the top half of the melodic minor scale.
In Bach’s chorale no.37, in A minor, the top four notes of the A melodic minor scale can be found in the bass line (E-F#-G#-A). The F# is harmonised with major chord IVb, D major.