Hi! Tyson Farmer with a quick rundown of the four types of 7 chords. This overview assumes you are familiar with your basic major and minor triads. Check out my "quick trick" forum post about triads if you need help with this:
https://www.lessonface.com/forums/piano-keyboard/126201-quick-trick-memorizing-your-major-and-minor-triads
First off, there are 2 main types of 7s you can add as single note chord extensions (not to be confused with the names of chords for this subject): the Major 7 or Δ7, and the flat 7 or ♭7. The traditional way to think about these chord extensions is to count UP from the root note to the seventh scale degree, but since the seventh scale degree moves up to the next higher root note in the scale, a much quicker and easier way is to count DOWN from the root as a shortcut to the seventh scale degree:
1 2 3 4 5 6 7 (1)
C D E F G A B C
^
The Major 7 is always a HALF step below the root note of a chord, and a ♭7 is a WHOLE step below the root note of a chord. Combine those two possibilities with the two main types of chords - the major and minor triads (I'm conveniently glossing over diminished and augmented chords since they're not as common or practical in everyday music writing), and you get FOUR main types of 7 chords:
MAJOR CHORDS:
"Major 7" chord: Major chord + Major 7 (1/2 step below root) - written "CMaj7" or "CΔ7"
"7" or "Dominant 7" chord: Major chord + ♭7 (WHOLE step below root) - Written simply "C7" - the ♭ is implied
MINOR CHORDS:
Minor/major 7 chord: Minor chord + Major 7 (1/2 step below root) - written "CmMaj7" or "CmΔ7"
Minor 7 chord: Minor chord + ♭7 (WHOLE step below root) - Written simply "Cm7" - the ♭ is implied
The third one, the "minor/major 7" chord, is pretty darn rare and deserves a side topic exploration, but the other chords, the Major 7, Dominant 7, and Minor 7 are extremely common and useful chords that you can be heard in jazz, pop, soul, funk, and all kinds of other music genres. There are other types of chords out there, but the 7s are the most common chords after the basic major and minor chords, and are the basis of most other advanced chords heard often in these genres.
Tyson Farmer
www.lessonface.com/TysonFarmer
