This is one of my all time favourite Christmas songs. A huge number of people have done this song but my favourites are Coldplay’s (in the video) and Patsy Monteleone’s version on MUD.
The chords in my version are loosely based on Patsy’s version – and, gosh, there are a lot of chords in it.
Some of the chord changes in this song are gorgeous. The change between C and Am7 is a fairly common one. You could play Am7 with all open strings but this one is much more effective. The chord is just the Gm shape moved up two frets (with the open G string giving the 7th note). It just goes to show what a huge difference the chord inversion you use can make.
The other stand-out chord changes come from very small changes in chords like the Em – Em maj7 – G progression. There’s only a difference between each chord of one fret of one note but it’s very effective.