Once you have more than one rhythm musician, however, you need some way to be playing the same chords. The simplest way, and the way most open bands do it by default, is to play the most standard chords, and play them each time through. I think this limitation is one reason why some people don't enjoy dancing to open bands as much. Another option is to practice alternate chords in advance. This doesn't work for open bands, and it also requires some amount of predicting in advance what you will want to do to a tune. My favorite option, though, is calling chords.

In calling chords you indicate what you're going to play before you play it. Generally by saying the chord name or number [1], though when I'm feeling particularly tounge-tied I'll sometimes point left or right with my head. Here's a clip of me calling chords when playing with the free raisins [2]. Playing with amy in the free raisins we've both been working on calling chords to each other, and I've gotten a lot better at it. Talking while playing is really hard for most musicians, including me, and thinking of what chord to play next long enough in advance to communicate it is hard too. This means that deciding to call chords was not enough, I had to practice it for a long time. And I still sometimes can't get the words out.

I played piano some at last sunday's bida dance with an open band, and did a bunch of calling chords. I think it went really well: we were able to play variations while moving together as a group. Through careful use of cheap tricks we got the dancers to shout a lot. It was really fun.

[1] If someone might be calling chords to you, it would be good to know your chord numbers. The idea is that no matter what key you're, in the chord with a certain feel is always the same number. You can figure out numbers from counting up from the root: in the key of D, D is the I (the one) chord. Then Em is the ii, F#m is the iii, G is the IV, A is the V, and Bm is the vi. The upper and lower cases indicate major vs minor, but chord calling is spoken and so major/minor is just implied. In fact, major/minor is also implied when saying the names of chords. If you're in D and someone says "E" they mean "Em". Sometimes even sharp/flat is implied, with "F" in the key of A meaning "F#m".

[2] It's not supposed to be audible to the dancers or end up in a recording, but I had my octave fiddle mic in the wrong place and when I leaned towards amy I was going right into that mic.