I spent some time writing down fourth chord guitar voicings on my flight back to the woodshed today. Fourth chords are especially fun on guitar because they're so easy to play. I've never practiced them enough to grab them on the fly while comping, but I want to get into that soon.

Here's the idea as I choose to apply it. Start with a scale that includes only consonant notes over a given chord. (See my earlier post on this topic for ideas on constructing such scales.) A fourth chord can be built on any scale degree, just stack fourths without straying from the scale (some will be augmented or diminished). I like the sound of two to four notes in these chords. More than four notes sound like too many different notes over too wide of a range. Fewer than two notes is not a chord. On guitar, I prefer playing these on the first four strings, sometimes the fifth, to keep from mudding up the low notes.

Fourth chords are most commonly used on modal jazz tunes to give the illusion of harmonic movement in an otherwise stagnant progression. Bill Evans and McCoy Tyner helped bring this technique into popular usage in the 1960s. Three-note fourth chords are easy to play with one hand on piano, even easier on guitar. The piano's big advantage is the ease of creating inversions of fourth chords, leading to cool-sounding second intervals. (Take a large fourth chord and displace the outer notes by an octave toward the center of the chord, condensing it to small, manageable intervals.) Fourth chords are easy to play on guitar because the strings are tuned to perfect fourths (with one major third); there's no wide stretching involved. Different fourth chords built on the same scale often have closely related, if not the same, shapes on the guitar.

My time-killing exercise on today's airplane trip involved writing out the fourth intervals for each degree of a few scales. I stuck with diatonic and melodic minor as most of the common consonant jazz scales are modes of these two. Below is what I wrote down and studied. In parentheses next to each scale degree is the type of fourth (perfect, augmented, or diminished) required to move up four scale degrees. Then, to construct a fourth chord, just stack these fourth intervals.

Diatonic

1 (4) 2 (4) 3 (4) 4 (#4) 5 (4) 6 (4) 7 (4)

Melodic Minor

1 (4) 2 (4) b3 (#4) 4 (#4) 5 (4) 6 (4) 7 (b4)

I also wrote out chord diagrams for all possible three-note fourths chords above, but I just need to use them and get them under my fingers. Can't wait.