Reddit reviews Elementary Rudiments of Music

I used Elementary Rudiments of Music when I first started learning. There's also a theory course available on Coursera.
Edit: Also if you're taking lessons most teachers also teach theory, though in my neighborhood they are usually separate lessons.

Wow, there is some great advice in this thread! Theory is not particularly hard, but we've imposed some frameworks on it that are efficient, but obscure.

The fact that major/minor, sharped/flatted, diminished/augmented all refer to adding or subtracting a semi-tone from a note, or that 'dominant' can refer to the fifth note in a major scale, or the fifth chord in a major key are at the root of the confusion.

There should be a guidebook.

It's wise to take the broader view and start with what you're currently playing. If you're playing open chord songs that consist of major and minor chords, learn what the distinct notes are for each chord. Then go look up what the notes are if you change that chord from a minor to a major or vice versa.

(All of the below examples are based on the A Major Scale, from which all of the chords in the key of A Major are derived: A B C# D E F# G#)

You'll find that only one note changes, and it's the note in the middle of the alphabetic sequence. We call that note the 3rd and you'll find that the minor chord has a 3rd that is a semi tone flatter than a major. Work that out for all the chords you know. Do that as you learn new chords.

Example: (The A major triad: A C# E, the A minor triad: A C E - see that we've 'flatted' the C# to a C)

You've probably learned that major and minor scales are seven tone scales. A lot of theory is based on what we do to those individual tones. Major and minor (triad - three note chords) are based on the 3rd tone of the scale (See example above).

As an aside, pentatonic scales are a subset of major/minor scales, where they've removed the two tones with the highest chance of clashing.

Diminished and Augmented chords are based on the 5th tone of the scale.
(Although we typically don't diminish three note chords, if we did, we might get; A minor diminished: A C Eb) I don't want to get into diminished too much - it's an outlier because there are subtleties.

Major 7th, Minor 7th, Dominant 7th are, you guessed it based on the 7th tone of the scale. And yes, there are 9th, 11th, 13th chords.

(Example: A Major 7th: A C# E G#, A Minor 7th: A C E G, A Dominant 7th: A C# E G)

We also have suspended chords where we might substitute the 3rd tone with a 2nd, 4th, or 6th.
(Example: A sus 2: A B E, A sus 4: A D E)


We might even just throw those rules out and have Major Minor 7ths.

So yeah, there's a lot going on, but if you try to learn as much about the current chord set you're playing, you'll find it starts to make sense.

Also, Barbara Wharram wrote a great book that Royal Conservatory teachers use as a theory primer called: Elementary Rudiments of Music. It's a very approachable workbook. https://www.amazon.ca/Elementary-Rudiments-Music-Barbara-Wharram/dp/0887970044/ref=sr_1_2/168-4402758-7301632?s=books&ie=UTF8&qid=1481846745&sr=1-2