A polychord is a "stacking", both literally and harmonically, oftwo "adjacent" chords within a key. For example, in the key of G, one of the seven possible polychordsis C/D. By C/D I mean a chord in which a C triad is played on the6th, 5th, and 4th strings, and a D triad on the 3rd, 2nd, and 1ststrings. One such way of playing this is (in psuedo-tab):
Where the 6th and 5th strings are fretting with the thumb, the4, 3 and 1 strings are fretted with a 1st finger barre, and the2nd string is fretted with the 2nd finger. (yes, it's awkward; work at it a bit. It's important that the 1st string ring outtoo; don't accept less than all 6 strings ringing).
Before we go futher with more polychord possibilities, you are bynow asking "what good is this"? It's a new tool. Play the polychordand listen. It's a unique sound, and creates a unique emotionalfeeling, which after all is the whole point of music, to createfeeling in the listener. Knowing polychords well gives you anopportunity to inject them in your music, be it compositions, beit free improvising, be it chord substitution when playing changes, etcetera.
Another interesting use of polychords is as a means of "inventing"interesting arpeggiated lead lines. By directly arpeggiating thepolychord tones, or otherwise using all six (or fewer) of themin close succession, your lead line starts to also take on the"mood" of the polychord. You get interesting intevallic leaps. (Be sure to base your polychord lead lines on a theme, with either a theme-variation or question-answer type of phrasing, as I'm sure you always do!! And of course you should probably resolve your lead linesto a chord tone of the actual underlying chord you are playing over. For example, someone's playing a D7. You go for a lead line usingthis C-D polychord. Ending on a D-F#-A-C will give more or lessresolution and sense of "yea, that fit's", while ending on a nonD7 chord tone will give alot more tension. Since your already creatingsome serious tension with the polychord already (playing a C over D7! ), you should probably initially work with resolving to a D7 chord tone.
Of course, a jazzer probably wouldn't call this a "polychord". They might view it as an inversion of D13, where the 7-9-11 areplayed "in the bass" and the 1-3-5 are played in the treble. Or, as a C with add 9, add 11, and add 13 (how do you write this, C+9+11+13? There is no 7, but perhaps folks would just write C13anyway? I don't know). Could be, could be. Viewing it as stacked triads ("polychords") is just a different mental model.
Another term for this concept is "upper structure triads", the ideathat the base chord is always a simple triad, be it major, minor, or diminished, and extensions are just triads "on top", such as5-7-9, or 7-9-11, or 9-11-13. Polychords we are defining here asbeing 9-11-13 played in the bass (strings 6-5-4), played over asimple triad 1-3-5.
Enough theory, on to more fun stuff! For playing lead stuff over polychords, whoa, you've got lots ofstuff to work with! Using our C/D polychord again, you canplay C lydian, D mixolydian (okay, the same notes but you are emphasizingdifferent chord tones), D blues. Hey, whole tone scales thrown infor a moment or two sound cool too!! Experiment. How many polychords are there in a key? Seven. The C/D in thekey of G is, from a theory/any key point of view, just IV/V. Okay, so the total set of polychords are:
I/II II/III III/IV IV/V V/VI VI/VII VII/I
Now how do we find them on the fretboard? Note that the 6-5-4 component of the C/D polychord is, of course, justa C triad in 2nd inversion (the 5th is a G, and it's in the bass, soit's 2nd inversion). Do you know all 2nd inversion triads in thekey of G on the 6-5-4 string set? I. e., staying with this form of5th of the chord on the 6th string, root of the chord on the 5thstring, third of the chord on the 4th string, play ALL the chords inthe key of G. We were just playing C at frets 3/3/2 on strings 6-5-4, so slide up to the D (the V chord) at 5/5/4. Now slide up to theEm on 7/7/5. The F#dim is at 8/9/7. And so on. Can you play all7 chords in the key this way, up and down the neck, in time witha metronome. No problem.
So now you have 1/2 of the 7 polychords. Next step: do the samework on the 3-2-1 string set. Our starting point was D, the V chord, in our C/D polychord. We used a 2nd inversion form (5th on 3, rooton 2, 3rd on 1). Next chord "up" is Em, which is at frets 4/5/3 onstrings 3-2-1, etcetera.
Now put these two chords together! The reality is, the fingeringis tough. Doable, but tough. You frequently need to use the trickof one finger fretting two strings.
Now the way out of the conundrum. Drop the 6th string out of theequation. You are dropping the 5th of the "top" (bottom? I don'tknow) polychord (in the C/D example, we are dropping the G, which isthe 5th of the C triad). That's okay, because we still have thethird (on the 4th string) giving us the "quality" of the chord (minor or major), and we have the root (on the 5th string).
Now work out the fingerings. They are quite reasonable, and we nowhave a whole new set of cool chords within the key, with a verydifferent vibe/feel from regular old major, minor, 7, 9 type chords. Try a 2 chord vamp using two of these polychords, record it for5 minutes or so, then jam over it. You got billions and billionsof scale and arpeggio possibilities to work with. Don't forgetwhole tone scales!! (I usually do, don't be like me). By the way, just to give you a sense that you indeed are on theright track, the Em/F#dim polychord, dropping the 6th string, is:
So that's polychords in a nutshell. As I started with, another toolto put into your bag.
Keep on jammin'!