What is up with the 7th Chord?

7th chords are very common in blues.

A7, D7 and E7 chords for example, are the staple diet of a blues rhythm guitarist. So much so that if they say "A" they probably mean "A7". Jazz musicians do this too.

In practice when building one of these chords on guitar, e.g. A7, we replace the 8th note, the A, with a G... which is a flattened 7 in A major. So that's just another hint that musical theory was written by drunks.

(The 8th note, the A, still "belongs" in the chord, but we sacrifice it, omit it, because we need to use that string to play the G.)

So I looked up "how to build a 7th chord"

And found this short instructional video....

So the chord with the flattened 7th is a "dominant" 7th (not a "Major 7", which would've had the non-flattened 7th). So "A7" (pronounced "A" by some blues rhythm guitarists) is A dom 7. It's also known as a "Major Minor 7th chord" which is perfectly COIK.

...then he goes through the 8 different kinds of 7th chord....

Any chord may also have a "sus" variation, what this means (e.g. sus2 or sus4)