This tune is in the key of G ionian, and therefore has G major, A minor, B minor, C major, D major, E minor and F# diminished as its chord options with G, C and D being the main ones. I always like to start out by breaking a tune down into “chord I sections”, “chord IV sections” and “chord V sections”. In the A part there aren’t any sections which really sound like they NEED chord IV. That means that most of the tune could have been accompanied by chord I (G), but I thought this was a bit dull, so I have thrown in a bar of chord IV (bar three), which works nicely because any time you have been playing chord I for a while you can always throw chord IV in for a bit of variation.
The final bar of each line has to be chord V (D). This is because the V chord always goes on every seventh foot tap, aka every fourth bar. You will notice that, unlike most tunes, McMahon’s actually has no V-I in the last bar of the A part (bar eight), but instead stays on chord V for the whole bar. The tune actually ends its A part on a long D note, and consequently sounds like it should stay on chord V instead of returning to chord I to make the section sound finished as most other tunes would.
McMahon’s is something of an unusual melody and can be quite tricky to find the right chords for. The B part changes key, to A dorian. A dorian contains the same notes as G ionian, but you can tell the mode has changed as there are suddenly far more As and Es (A dorian’s root and 5th) on dominant beats of the melody, making the section feel a lot more A minor-y and a lot less G-y! In the dorian mode the main chords are chords I, IV and V as normal, but chord V can also be replaced with its related major if you prefer a less dark sound. For that reason I have broken this part of the tune into chord I (Am), chord IV (D) and chord VII, which is chord V’s related major (chord VII is G, the related major of E minor). You can gain a much better grasp of how and why chords fit together using the Amazing Mode Wheel, available here.
As we are now in the key of A dorian, I have begun the B part on A minor. The third bar is pretty vague in its tonality- its notes are “E B B B G – F# G”. To me this sounded most like a G chord (G, B and D) but really I’m never too sure with this tune whether there is really any right answer! The fourth bar is a bit more clearly a G major section; “E B B A B – – -” . We would expect a chord V in this bar for our seventh foot tap, and chord V (Em) in the dorian mode can always be substituted to its related major chord VII (G) for a lighter sound, so another bar of G is a good fit.
In the B part’s final bar, the tune is transitioning back to being in G ionian. For this reason I have ended the section on chord V from the original key of G ionian, aka D major. This makes the transition back to G seem natural.
In this version, I have added a G/B chord, otherwise known as G in the first inversion. This is a very common technique, which creates a nice linking bassline from G up to B and then on up to C. A G major chord normally contains the notes G, B and D. It doesn’t matter what order you pile the notes within the chord, but it is traditional to have the root note at the lowest pitch. For example G, B, D, G, B, G or G, D, G, B, D, G would both be valid G major chord voicings. If you invert the chord so that its 3rd, B, is at the lowest pitch within it then the chord is said to be “in the first inversion”. To play chord I, chord I’s first inversion, then chord IV is a very common technique in folk backing and will work in any key.
In bar six of the A part I have replaced G major with its related minor, E minor for a darker variation. Any chord can be replaced with its related minor at any time, unless it is chord V in a major key which has to be chord V and cannot be substituted.
In the B part, I have capitalised on the vague tonality of the tune to put in a more “pop music” type chord progression. For example the second bar contains the notes “D B B A B – – D”. This could be seen as the top of a G major chord with an A thrown in, so I’ve stuck a G in there instead of the A I went for in the previous version. The third bar contains; “E B B B G – F# G” which is so vague as to be basically a free for all. I decided to go on round the circle of fifths from G to C as progressions which go up in fifths always sound nice and this is basically a chord I section which means you can put anything from the chord I or IV families here without any adverse effects!
Basically there are so few clearly defined chords in this section (each bar contains notes from multiple chord families) that I have simply plumped for a chord progression which sounds good in its own right, and more or less fits with the melody.
In this version I have converted each of the chords from the substitutions version to a jazzier version of the same. When adding tetrads (four note chords) or more complicated “jazz” chords into folk music in major keys, chords I and IV become major 7 type chords, chords II and VI become minor 7 type chords and chord V becomes a dominant type chord. You can learn a lot more about how to implement these types of chords in my book Backing Guitar Techniques for Traditional Celtic Music.
Therefore in my version, G major becomes G69, the quintessentially gypsy jazz sounding substitute for a major chord (replacing chords I and IV in a major key with 69 chords always sounds nice). In bar two, B minor is a substitute for G major, because a major chord can be substituted for the minor chord two degrees below it within the key scale (its related minor- in this key G is related to Em) or the one two above it in the root scale (in this case G can sub to Bm). As we are in jazz-chord territory I play Bm7 instead of plain Bm. As the following bar’s Am has become A minor 7, and this chord is an identical barre chord to Bm but two frets lower on the neck, I have slid down and played the intermediate chord on the way to provide a nice chromatic link, Bm7 – B♭m7, Am7. Finally I have used D9 in place of chord V, because chord V always becomes a dominant 7 type chord when you get to jazz land, and adding any extra notes from the root scale always makes a jazz chord jazzier! In this case a D7 chord would contain D (root), F# (major 3rd), A (5th) and C♮ (flattened 7th). The 9th note in a D major scale would be E, and E is also in our key scale of G ionian so adding an E note to my D7 chord to give me E9 sounds good. As a general rule, using a 9 chord (D9, A9, etc) in place of chord V in a major key pretty much always sounds great.
In the B part, I have more or less taken the chords from the basic version and converted them into the right kind of tetrad. Recall that the chord options for A dorian are the same as the ones in G ionian. This applies in tetrad land too, so the complete chord options are G major 7, A minor 7, B minor 7, C major 7, D (dominant) 7, E minor 7 and some sort of substitute for F# diminished. As we are in A dorian, chord I is Am7, chord IV is D7 and chord V is Em7.