7ths are tricky to notate
7ths are a problem in music theory because we have two completely different systems, and they conflict. In traditional theory, which unfortunately still dominates as the system most frequently taught, the 7th note of any scale is 7. The lowered 7th is m7 (minor7).
In the chord system, that regular 7 is always maj7, and the lowered 7th is simply 7. I always try to mark 7 as “(b)7” to remind people that you don’t need the “b”. The “(b)” is always there to tell you to lower that 7th. You have to know which system you are using in order to understand what is going on.
- maj7 = M7
- (b)7 = m7
- bb7 = dim7
Going into more depth:
In the traditional system the 7th note of a major scale is always like this:
C D E F G A B C: 1 2 3 4 5 6 *7 8
But in X chords, the 7th note is always maj7 for X maj7. This is the first conflict, like this:
C D E F G A B C: 1 2 3 4 5 6 *maj7 8
Then when the 7th note is lowered, it is marked m7 for minor 7th in the traditional system, like this:
C D E F G A Bb C is 1 2 3 4 5 6 *m7 8
In chords it is always this way:
C D E F G A Bb C is 1 2 3 4 5 6 *7 8
In chords 7 is ALWAYS b7.
This is the confusion. To get around this problem I have carefully marked these two kinds of 7ths with maj7 and (b)7.