Amateur guitar player trying to determine the method behind chord structures.
The difference between E, Em, and E7... what "add9" or "C/G" means in relation to the way I should play.
Any thoughts? Keep in mind that I don't have much in the way of formal training.

Guitar aint my thing unfortunately...:(

Only piano :)

Er, if you haven't looked into scales (in diatonic space), and how they're constructed, why a major scale is major, and a minor scale is minor, then you really should get that under your belt.

A scale is just a series of notes, where each successive note is higher (lower) in pitch. There are a lot of different scales, but maj and min are the most used.
You know how to form a minor third from a major third? That's how to construct the start of a minor scale. Minor scales have a different set of notes on the way up, than on the way down the scale.

A 7th is just the 7th note in a scale. A min 7th and a maj 7th are different because they're constructed from a minor and a major scale. There are 9ths 13ths, and so on.

Every major scale has a relative minor scale. Cmaj, for instance, has a relative minor (Amin), which is the minor scale with no sharps or flats (like Cmaj), and A is the 6th note in the scale of Cmaj; it's also the note which is a min 3rd below C.

If you see "add 9", that means add the 9th note in the scale to the chord (sometimes you see "D bass", or some other note - which means add a low "D" to the chord). In the scale of Cmaj, the 9th note is the 2nd note of the 2nd "Cmaj octave", i.e. "D", so Cmaj add 9, is Cmaj + D. If you play C + D anywhere on a piano or guitar, that's a C9.

Get a music theory book and read up on intervals and harmony - Trinity College publishes good ones. Start with Grade 1.

The guitar is probably the most difficult instrument to learn music theory on.

It might be easier to learn enough piano to get the theory on that, than learn the theory on the guitar from scratch. Even sight reading notes on a guitar is a hassle for a beginner.

I'll look around for the lessons published by Trinity College. I also have a piano, so if the consensus is that learning music theory is easier while playing the piano, and then later applying that knowledge to the guitar, I'd be glad to learn that way. I've always wanted to learn how to play the piano... more than what I currently do, which is just striking the middle C key and starting off on Ode To Joy or some boring melody. Do you know of any free publications available on the internet for the piano or the guitar? I wouldn't mind paying for an outstanding guide, but I'd rather pay for something I can understand instead of just getting confused and squeezing it onto my bookshelf.

Well. I never really got into books myself. I took lessons instead, but that was a long time ago.

Most of what I know about theory is in the first half of the Trinity College Grade 1 handbook.

You can figure a lot out all by yourself, once you get the hang of scales, and a piano is a more obvious kind of layout.
I can read music, but I remember I hardly could years ago when I got back into playing after about a decade of neglect. It comes with practise.

Constructing scales gets easier once you understand the symmetry, If you know where the scale of C is, then you already have eight notes to start other scales from (with or without sharps/flats), and they're all valid forms (dorian, ionian, etc).

But, yeah major and minor are the first two significant ones, and then there's augmented, diminished, and suspended intervals, inversions (like how C to A is a 6th, but A to C is a min 3rd), and so on.

There's a built-in symmetry in the 12-note semitone scale, you have to stuff around with it to see what it does.
Try, as an exercise, playing the scale of C, and then repeat starting on D, and subsequently starting a scale on each successive note in the scale (of Cmaj), and listen to how each scale sounds.

sorry i am learning the drums!

i am learning the drums!Right.
Drums have a tone or pitch too.
A snare or high-hat is a high "tone", and a bass drum (or a kettle drum) has a low one. Then there are all those "medium" tones.

Of course, a single tone (note) is really a whole group of notes (you can analyse a note from any instrument, including a drum, and find a whole bunch of overtones or harmonic parts of any "single" note).

You can put a tune together or play a scale of sorts on a drum kit, too.
There are atonal "melodies", as well as chromatic or diatonic. Isn't sound amazing?