The story
From Gradus ad Parnassum (Latin, literally, a step to Parnassus), a 17th-century prosody dictionary long used in British schools. Doublet of grade.
Etymology from Wiktionary, CC BY-SA 4.0.
gradus
Today it means: A handbook used as an aid in a difficult art or practice, specifically, a dictionary of Greek or Latin prosody used as a guide in writing of poetry in Greek or Latin. Full definition →