Section 26.3 Objectionable Parallels
When writing in three or four parts, avoid having any two voices move in parallel fifths or parallel octaves (and their octave equivalents—parallel twelfths and parallel unisons). In the Renaissance, Baroque, Classical, and Romantic eras, these objectional parallels were thought to destroy the independence of the voices. Parallel 3rds, 4ths, and 6ths were considered acceptable. Parallel 2nds and 7ths generally were not used.