Abbreviation for Soprano I, Soprano II, Alto. Usually found on choir music, indicating which voices the piece is arranged for. SSA will be for an all female choir or a boy's choir, with the soprano section divided in to two parts, one high and one lower, or mezzo-soprano section. Often the two will sing together, separating only in parts. (cf: SATB, SAB) Commonly a piece will have multiple arrangements appropriate for different groups - sometimes in different keys and with the melody in different voices to accommodate the different vocal ranges of the four voices, so choosing the correct one is very important.