Ok, i think im pretty confused with this stuff, i hope to make it as easy to understand as possible. I wanted to play a simple 12 bar blues using the common progression, I, IV, V (right?). I was looking to make a dominant 7th chord. I beleive the formula for that is I-III-V-bVII. Correct?

That would make..
C7= C, E, G, Bb
F7= F, A, C, Eb
G7= G, B, D, Fb

Wouldn't Fb be E? (enharmonic?)

So, if i take a major triad and add a flat 7th, does that make it dominant 7th?

If i take a minor triad and ad the flat 7th, doesnt that make it a diminished 7th?

I would like chord forumlas for lots of chords please, if you dont mind.

And just making sure, you use the chromatic scale to figure out scales and the major scale to figure out chords, right?

I'm so lost... thats what i understand, but im sure there are flaws all over it. Thanks for the help!
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the G7 chord is spelled G-B-D-F

since in the key of G there is an F#, flatting that F would result in an F natural, not Fb
Dominant 7th chords are 1 3 5 b7, correct. However, that is applied to the key of the chord, not the key of the tonic chord. Your G7 is incorrect, it's GBDF because in G, F is sharp. Flatting it (the 7th of G) would make it F natural.

A major triad with a the flat seventh of its root is a dom7 chord. A minor triad with the flat seventh of its root is a minor 7 chord. A diminished 7th chord has these intervals: 1 b3 b5 bb7
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Quote by 812many

So, if i take a major triad and add a flat 7th, does that make it dominant 7th?

correct

If i take a minor triad and ad the flat 7th, doesnt that make it a diminished 7th?

no, it makes it a minor 7th. Also there are two types of diminished 7th chords, full and half dim. adding a m7 interval to a diminished chord would yield a half diminished chord, while adding a º7 interval would yield a full diminished.

so, the formula for a minor 7th chord is
1-b3-5-b7

the formula for a half diminished 7th is
1-b3-b5-b7

and the formula for a fully diminished 7th chord is
1-b3-b5-º7 (or bb7)