Also from ECTA's definition:

"Contra Dance is mainly danced in two long, facing lines of couples, the longways. There are different setups relating to the position of the partner in the formation (e. g. Proper, Improper and Becket) and relating to the number of couples dancing together (Duple, Triple, Triplet). A different formation is the Sicilian Circle. Facing couples create a ring in which in each group one couple is dancing counter clockwise and the other clockwise.
Contra Dances are mostly danced to reels and jigs, but also to hornpipes. The music consists of the melodies A and B, which each contain 16 bats and are repeated so that a melodic structure of AABB is created. The set of figures is danced to this AA BB structure of 64 beats. There also are deviant structures like AB BA.
The Prompter explains each dance and calls the figures to the music a little ahead of time. Like this it is possible for both experienced and new dancers to dance. Through the repetition of the set of figures the dancers learn the dance and the Prompter can shorten his calls or reduce them bit by bit. The set of figures is written so that after dancing through it once a new group of dancers meet. This is called the progression. In formations with a fixed group of couples the position within the formation changes. "[sic]

Bob