Instant Theory: The One-Hour Crash Course for Guitarists

In this lesson, learn enough theory to get you round the bases. And it only takes one hour!
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Theory is often viewed as a set of strict rules governing what can or can’t be done in music. But that’s a somewhat inaccurate way of looking at it. More than anything, theory is an evolving system that provides a means of not only interpreting musical ideas but also communicating with other musicians.

A grasp of basic theory will make your overall playing experience more rewarding.

And guess what? It’s really not as difficult as you think. In this lesson, we’re going to learn enough theory to get you round the bases. And it only takes one hour!

Plus, we’re providing plenty of tabbed examples with audio files to make it even easier. 

So grab your guitar and get ready to learn. 


Music theory starts with intervals and scales. An interval is the distance in pitch between two notes.

FIGURE 1 shows a series of increasing intervals all using C as the reference pitch. The interval names—starting with the first example (one half step, or one fret) and moving onward—are as follows minor 2nd, major 2nd, minor 3rd, major 3rd, perfect 4th, augmented 4th (diminished 5th), perfect 5th, minor 6th, major 6th, minor 7th, major 7th, and perfect octave.

Abbreviations for each interval are shown below the staff. Lowercase m indicates minor, while an uppercase M references major. The letters p, a and d stand for perfect, augmented and diminished, respectively.


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By ascending in a series of half (H) and whole (W) steps—that is, minor and major 2nds—we can construct the C major scale (FIGURE 2A) and the C natural scale (FIGURE 2B).


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Located above the staff here are numerical scale degrees. Note the flatted 3rd, 6th and 7th tones of the natural minor scale. These degrees are a half step lower than the 3rd, 6th and 7th tones of the major scale.

When the C natural minor scale was shown in FIGURE 2B, symbols for the flatted notes were placed on the staff as needed. Key signatures eliminate the need for such accidentals (sharps and flats) by indicating that certain pitches remain sharp or flat throughout the arrangement unless another symbol indicates otherwise. The proper key signature allows for notes to be written without the clutter of repeated accidentals.

The circle of 5ths (FIGURE 3A) represents all the keys. At the 12 o’clock position are C major and A minor. Neither has any sharps or flats. Moving clockwise brings us the interval of a perfect 5th higher, to G major/E minor. Each contains one sharp. Each clockwise step adds one sharp to the key signature. Counterclockwise from C/A minor, the circle ascends in 4ths, introducing one flat with each step.


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Since the keys in each major/minor pair share a signature, they are known as relative keys. For example, A minor is the relative minor of C major, and Bb major is the relative major of G minor. The three lowest points along the circle are where flat and sharp keys converge. At these points sit enharmonic keys, for which two possible signatures represent the same notes. FIGURE 3B shows all of the key signatures as they appear on the staff.


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By stacking 3rds to form blocks of scale tones, we can find chords that are diatonic (native) to a given key.

FIGURE 4A contains all the diatonic triads (root–3rd–5th) in the key of G major. The Roman numerals below the staff are a shorthand method of indicating chords. Each numeral represents the scale degree of the chord’s root—uppercase denotes a major chord; lowercase, a minor chord. The diminished symbol (˚) is added to the vii numeral to indicate that the 5th is flatted.

FIGURE 4B is a diatonic ascent in Eb major. The numerals remain the same as they were in the G major example, since they represent the same chord qualities and scale degrees.


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FIGURE 4C moves through the chords in C minor. While the same chords are found in Eb major, the Roman numeral symbols differ. Why? Because the tonic is no longer Eb but rather C. The C, then, has switched from the vi chord to the i chord.

Stacking another 3rd onto each triad gives us all the diatonic 7th chords, as seen in FIGURE 4D. Here, the 3rds of each chord have been moved up an octave to provide for more guitar-friendly voicings.


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A cadence is a point of rest in a piece of music. The dominant (V) chord has a strong pull toward the tonic (I) chord because the leading tone—here (FIGURE 5A) the 3rd of the V chord (G) and the 7th note of the major scale relating to the I (C)—literally leads the ear back to the tonic, located a half step higher.

You can add further tension by making the G a G7 (FIGURE 5B), the 7th (F) of which resolves down to E, the 3rd of C. This V7–I move is known as a perfect cadence.

In contrast, a IV-I move (FIGURE 5C), called a plagal cadence, sounds less suspenseful that the V-I.

Approaching the V chord from any other chord, as in the ii–V sequence of FIGURE 5D, results in an imperfect cadence, which maintains some tension by delaying resolution to the I chord.


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A perfect cadence in a minor key requires a slight change to the v7 chord. The b7 of the natural minor scale is raised a half step to create a leading tone, turning the v7 into V7 (FIGURES 6A–B). Since the harmonic minor scale (1 2 b3 4 5 b6 7) contains a leading tone (the natural 7th), it works well for soloing over the V7 chord in a minor key.


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When a chord tone other than the root is the lowest note, the chord is an inversion.

In 1st inversion, the 3rd is the lowest note. In 2nd inversion, the 5th is the lowest note. And in the case of 7th chords, 3rd inversion positions the 7th at the bottom.

Inversions not only give chords a rich sound but also allow for different types of movement in bass lines. In FIGURE 7, a bass melody jumps down and then climbs back up to the tonic, all via a handful of inversions.


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Another way to add harmonic variety is through chord substitution. In FIGURE 8, a non diatonic D7, substituted for C on beat 2, acts as a secondary dominant, often call a “V of V.” While G7 is the actual dominant chord in the given key (C), D7 (the V of G) is used to provide a strong sense of movement toward the G7, which in turn resolves to the tonic, C.


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Dominant 7th chords can also help reinforce the sound of key changes, as in FIGURE 9A–B. Despite being nondiatonic at the moment they appear, the chords preceding each key change end up sounding proper as they move smoothly into the new key.

Tritone substitution, meanwhile, involves replacing a dominant chord with one whose root is a #4 (or b5) away. In FIGURE 9C, a D7 chord is used in place of Ab7 to set up a key change to Db. The 3rd and b7th of the D7 chord, F# and C, are the same pitches as the b7th and the 3rd of Ab7, Gb (F#) and C. In FIGURE 9D, a simpler example in the key of E, B7 is replaced with F7.


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Taking a scale and beginning at a point other than the tonic produces a mode. Within the C major scale,

C to C = Ionian mode (the major scale)
D to D = Dorian
E to E = Phrygian
F to F = Lydian
G to G = Mixolydian
A to A = Aeolian (natural minor)
B to B = Locrian

FIGURES 10A–E show these modes (minus Ionian and Aeolian) with C as the root. (In FIGURE 10F, you’ll find the exotic Phrygian dominant scale, the fifth mode of harmonic minor.)


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Of course, those modes get some notes from outside their keys. For example, C Dorian contains the same pitches as Bb major but uses C minor as its tonal center. Finally, FIGURE 11 is a short C Mixolydian (C D E F G A Bb) vamp in which chords from the key of F major are again applied to a tonal center of C.


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That’s all for this lesson. Hopefully you have some useful tools to help you understand musical ideas as well as communicate them with others. Keep studying these lessons until they become second nature, and you’ll find your musical knowledge and ideas expanding and growing stronger with time.