You do not need theory to play guitar. Plenty of legends learned entirely by ear. But a little theory is like a map: it explains why the chords in a song fit together, helps you learn songs faster, and makes soloing far less of a guessing game. This guide covers the practical essentials in plain English, always tied to the fretboard.
The Musical Alphabet
Western music uses just seven letter names: A B C D E F G. After G, it starts again at A.
Between most of those notes there is an extra note, a sharp (#) or flat (b). A# and Bb are the same pitch, named differently depending on context. The two exceptions are crucial:
- B to C has no note between them.
- E to F has no note between them.
That gives twelve notes in total before the pattern repeats:
A A#/Bb B C C#/Db D D#/Eb E F F#/Gb G G#/Ab (A)
The Fretboard Is a Ruler
On the guitar, each fret is one half step (one step along that list of twelve). Two frets is a whole step. At the 12th fret, the note repeats an octave higher, which is why many guitars have a double dot there.
Notes on the low E string
Fret: 0 1 3 5 7 8 10 12 Note: E F G A B C D E
Notes on the A string
Fret: 0 2 3 5 7 8 10 12 Note: A B C D E F G A
Learn these two strings first. They tell you where to put the root of every movable power chord and barre chord. Quiz yourself: name a random note, then find it on both strings.
Intervals: The Distance Between Notes
An interval is the distance between two notes. A few matter most:
- Octave (12 frets): the same note, higher.
- Perfect fifth (7 frets): the two notes of a power chord. Find it one string over and two frets up.
- Major third (4 frets): the note that makes a chord major (bright).
- Minor third (3 frets): the note that makes a chord minor (dark).
That single fret between a major and a minor third is the difference you hear when you switch from E to Em: one finger, one fret, a completely different mood.
The Major Scale
The major scale is the "do re mi" sound, and it is the yardstick for everything else. It follows a fixed pattern of whole (W) and half (H) steps:
W W H W W W H
Starting on C gives you C D E F G A B C, with no sharps or flats. Starting on G and following the same pattern gives you G A B C D E F# G. The pattern forces the F to become F#.
How Chords Are Built
Most chords are built by stacking every other note of a scale. Take the 1st, 3rd and 5th notes of the C major scale, C, E and G, and you have a C major chord. Do the same starting on the 2nd note, D, F and A, and you have D minor.
Do this from each note of the scale and you get the seven chords that belong to a key. In any major key, the pattern of chord types is always:
I ii iii IV V vi vii° Maj min min Maj Maj min dim
In the key of G, that means:
G Am Bm C D Em F#dim
Notice anything? G, C, D and Em, four of the friendliest open chords, all belong to the key of G. That is why they sound so good together, and why the G-D-Em-C progression from Your First Guitar Chords works.
Progressions as Numbers
Musicians often describe progressions with Roman numerals so they can be moved to any key:
- I - IV - V: the backbone of the blues and early rock and roll. In A: A, D, E. See Blues Guitar Basics.
- I - V - vi - IV: one of the most common progressions in pop. In G: G, D, Em, C.
- ii - V - I: the bedrock of jazz. In C: Dm, G, C.
Learn a song's progression as numbers and you can transpose it to any key in seconds.
The Pentatonic Scale
The minor pentatonic takes five notes from the natural minor scale and drops the two that are most likely to clash. That is why it is so forgiving for improvising, and why it dominates blues and rock soloing.
In A minor pentatonic, the notes are A, C, D, E and G. Interestingly, these are the same notes as C major pentatonic, just starting from a different place. This idea, called relative major and minor, means every major key shares its notes with a minor key three frets below. C major and A minor; G major and E minor.
Keys and the Capo
A capo clamps across the neck and raises the pitch of every open string. Put a capo on fret 2 and play a G shape: the chord that sounds is A. Capos let you keep the open shapes you know while matching a singer's range or a recording's key.
How to Use This Without Getting Overwhelmed
- Learn the notes on the low E and A strings this week.
- Next week, work out the chords in the keys of G and C.
- Then, each time you learn a song, figure out its key and write the progression in numbers.
Theory sticks when you use it. Ten minutes applied to real songs beats an hour with a textbook.
Next Steps
Put theory to work with power chords in Rock Rhythm Guitar and the pentatonic in Blues Guitar Basics. For a deeper look at how great players think about the fretboard, read about how Joe Satriani became one of guitar's greatest teachers.