Chord Theory

Here's some basic rules about guitar chord formation which I've picked up while attempting to learn how to play guitar:

Major triad chords (i.e the basic chords) are made from the root note of a Maj scale + the 3rd & 5th notes of that scale, for example, from the C Major Scale (C, D, E, F, G, A, B, C), you make a C Major triad chord by playing the root (C), the 3rd note (E), and the 5th (G). Depending on the chord, sometimes the 3rd note is a natural (e.g. E, as above), sometimes a sharp, as with the AMaj chord, in which the 3rd note is C-sharp (C#).

Chord transformations:

To turn a Major chord into a minor one, lower the 3rd note by a semi-tone. If the 3rd note is a natural (e.g., E), lower it to a flat (Eb); if it's a sharp (e.g., C#), lower it to a natural (C) by moving it down one fret.*

To turn a Maj into sus4 (a suspended 4th chord), raise the 3rd note (i.e. whichever one is moved down one to change a Maj into a min), by a semi-tone (up 1 fret),

To turn a Maj into sus2, lower the 3rd note by 3 semi-tones (usually 2 frets)**.

Chord transformations for barre chords (i.e. where first finger presses down on all the strings below the chord formation, so that there are no open strings):

As above, except whatever shape is made can be held & simply moved up and down the board to make the equivalent for other chords, because the barre means that all notes not just the open ones will also shift up or down, and an equal raising or lowering of frequency is what is required to turn one chord into another. On the downside, as this requires the first finger used as the barre, it can only be done for chords that can be shaped with 3 fingers above the barre. Because what applies to one applies to all, this can be memorized in terms of finger changes rather than note alterations:

lift 1st finger to make minor,
lift 3rd finger to make 7th,
lift 1st & 3rd to make minor 7th,
put all 3 fingers in the 2nd fret up from the barre to make sus4

*with B-C and E-F changes, one fret = one whole tone, there's no inbetween, but if you're starting from a sharp, which by definition is the mid-point between notes a semi-tone above and below, you know that one fret up or down will always be a semi-tone; if the 3rd is a natural note, it will always be one which can either be sharpened or flattened, so again, it will be one fret up or down.

**this is why Esus4 can easily be made, but not Esus2, cos with the E chord, unlike with A and D chords, the 3rd note is made on the 1st fret and there's only one semi-tone down to the open chord to go from there (with A & D, the 3rd note is on the 2nd fret and can go 1 semi-tone down to make minor, and with these notes there's then a whole tone leap to the open chord).