What is "gradus ad parnassum"

Latin, literally "A Step to Parnassus," mountain sacred to Apollo and the Muses, title of a dictionary of prosody used in English public schools for centuries as a guide to Roman poetry. The book dates from the 1680s.

The Latin phrase Gradus ad Parnassum means "Steps to Parnassus" (see below). It is sometimes shortened to Gradus. The name Parnassus was used to denote the loftiest part of a mountain range in central Greece, a few miles north of Delphi, of which the two summits, in Classical times, were called Tithorea and Lycoreia. In Greek mythology, one of the peaks was sacred to Apollo and the nine Muses, the inspiring deities of the arts, and the other to Dionysus. The phrase has often been used to refer to various books of instruction, or guides, in which gradual progress in literature, language instruction, music, or the arts in general, is sought.