I've been on country jobbing dates (pickup gigs) where the leader(usually the singer or bass player) would walk the group through the progression using his/her fingers.

Once the key and style is stated ("It's a waltz in C") you signal the chord changes by extending the appropriate number of fingers. Fingers pointed up are major chords; pointed down minors.

That's about as sophisticated as we got; after going through the entire progression once or twice some people would start adding those 7th's, 9th's, argumentative and demolished chords if they wanted. As long as people knew the drill and were paying attention, you could do a whole lot of songs you never even heard before using this system.

Fingers - or I should say, one specific finger - was also used to signal that your playing wasn't measuring up.