First Grammatical Treatise

The First Grammatical Treatise (Icelandic: Fyrsta málfræðiritgerðin) is a 12th-century work on the phonology of the Old Norse or Old Icelandic language. It was given this name because it is the first of four grammatical works bound in the Icelandic manuscript Codex Wormianus.

Source: Wikipedia — First Grammatical Treatise (CC BY-SA 4.0)