_Fourth._ A. D. 384 (about).--St. Ambrose brought together the inharmonious elements in the church liturgy and formulated a general system of chanting known as the Ambrosian chant. The foundation of singing-schools produced choristers who were able to meet the requirements of the improved music, for that was beyond the narrow powers of the early congregations. The reader will readily see how the first three steps toward the formation of a system were logical. But differences in practice naturally crept in, and the work of Ambrose appears to have been one of regulation. He founded his system on four of the ancient Greek scales, which were, of course, at the base of all the Greek and Roman tunes then used in the Church. It is unnecessary to go into any extended account of Greek music in order to get an idea of the character of the Ambrosian chant, but it is needful to give the subject some consideration, because Greek music influenced modern music for several centuries. All modern major scales are formed thus: two whole intervals followed by one-half interval (a semitone), then three whole intervals followed by a half. For example, take the scales of C and G:-- [Music C scale: 1 2 ½ 1 2 3 ½] [Music G scale: 1 2 ½ 1 2 3 ½] The Greek scales were formed on a wholly different principle. The foundation of the system was the tetrachord, which always contained, as its name implies, four notes. Between some two of these there was always a half-interval, and the scale was named according to the position of that semitone. The Doric scale had the semitone at the beginning of each tetrachord, the Phrygian in the middle, and the Lydian at the top, thus:-- DORIC. [Music: ½ 1 2 3 ½ 1 2 3] PHRYGIAN. [Music: 1 ½ 1 2 3 ½ 1 2] LYDIAN. [Music: 1 2 ½ 1 2 3 ½ 1]