Major 7th and Seventh Chords
There are two different major chords described as sevenths in the diatonic scale.
C major 7th or Cmaj7 and a C seventh or C 7.
In creating Cmaj7 from the C Major chord we flatten the C one semitone to B.
In creating C7 from the C Major chord we flatten the C two semitones to Bb.
There can only be one 7th chord that is a Major in the sense of C7 in a key. So that chord will determine the key of the bar
and possibly the piece in which it appears.
There can be more than one Major 7th chords in a key, as in Cmaj7,Fmaj7.
The relationship of the notes means that often the scale can contain chords, embellished, with the following attributes.
C Major Chord Progression
C, Dm, Em, Fmajor, G7, Am, Bdim, C
or an embellishment
Cmaj7, Dm7, Em7, Fmaj7, G7, Am7, B half dim, Cmaj7