Ever feel like you've suffered from a Brain-Freeze just trying to understand a little theory after reading the first few lines some music Professor wrote? It's like reading R2D2. The fact is that the theory section of this forum has the least threads to offer some stepping stones that could cause the light bulb to flicker for someone. Understanding theory is manditory for any type of serious musician, no matter what you play. Many players learn chord structures, certain scales and techniques but never grasp how they relate to the music they're playing. A friend of mine has been playing guitar for years and plays great, but doesn't understand why a simple chord progression would use a C Major, Dm, Em and G Major in a verse. His solos are weak to say the least because of it unless he's doing a copy and figures most notes by ear.
You should have some basic knowledge in music before diving into theory, like knowing the intervals to construct a Major scale. That's where it all begins.
Starting with the Major scale helps to understand how notes relate to each other. Theory is better understood from the start. Music starts and ends with the C Major because it contains no accidentals (no sharp or flat notes) which are C-D-E-F-G-A-B, a total of seven notes. Each note is numbered,
C-1, D-2, E-3, F-4, G-5, A-6, B-7. Every Major scale will contain 3 major notes, 3 minor notes and 1 diminished note and are always in the same order from the root note. 1-major, 2-minor, 3-minor, 4-major, 5-major, 6-minor and 7-diminished. So the 2nd note of C Major is Dm. Why? For the answer you must go back to the Major scale. Since the note is D we use the D Major scale. Notice the third note in D-Major is F#. Flatten the third note 1/2 step makes it F, causing D to become minor and F fits in the C-Major Scale. Try the other notes and it will help to understand what chord progressions and scales fit well together. I call it the OJ theory "If it doesn't fit, you must aquite" because it allows you to bend the rules sometimes.
Next up> The circle of fifths.