Chords are presented on the left as the letter of each chord, for the tune in a specific key. On the right, the same chord progression is presented using chord numbers, where 1 is the root chord of the key you play the tune in and the other numbers correspond to the relative chords in that key. The numbering system lets you learn a song once, and then you can play it in any key you'd like.

Some songs have more than 4 bars per line, so the extra bars are presented with () to indicate, "Play an extra bar beyond your 4 bar intuition for these kinds of songs."

When a chord changes within a bar, the chords are presented as 2 chords separated by a slash, so you play each chord for half a bar each. This notation is may be misleading to folks who are used to standard sheet music notation, where C/G indicates "G chord with a C bass note". DhMusicCharts are intended for jam circles where everyone knowing the chords and timing is most important, and each player is free to play the chords any way they want and with any root note. So I think I will stick with my notation as it effectively communicates the timing for jam circles.

Occasionally, a song that is in 4/4 timing will have a 2/4 bar thrown in somewhere. I present that kind of chord change with the chords letter followed by a slash, because when playing a 2/4 bar in a 4/4 song, you essentially are only playing the first half of a 4/4 bar before moving on to the next 4/4 bar.