The Trio Sonata, in it's various manifestations, came to be the most popular instrumental form at the close of the seventeenth century and in the first half of the following century, only superseded, in course of time, by the classical string quartet. It represented an ideal economy of means, in that it needed minimally only three or, more usually, four performers, while capable of expansion into a full concerto grosso by the addition of ripieno players to reinforce the louder sections. As it developed the Baroque trio sonata came to encompass two generally distinguishable categories of work, the Sonata da chiesa or Church Sonata, with it's alternation of slow and fast movements, the latter generally fugal in character, and the Sonata da camera, a suite of dance movements. Most commonly the trio sonata demanded the services of four players. Two melody instruments, normally violins, although publishers allowed some latitude in the matter, however unrealistically, were supplemented by a bass melody instrument and a chordal instrument in the form of a harpsichord, organ or lute. It was, however, always possible to play trio sonatas without chordal filling from the keyboard or it's equivalent. Published music sometimes described the second violin part as optional, although such an omission would normally be impossible. Generally trio sonatas would be issued with only three part-books, the third to be shared by keyboard-player and player of the viola da gamba, cello or violone. In texture they might differ between sonatas in which each melody instrument held a contrapuntal line and sonatas in which the lowest instrument simply provided a harmonic basis for melodic interchange between the violins, or a close shadowing of the first by the second.