praeceptum

Praeceptum, praecepti is second declension neuter, the perfect passive participle of praecipio, "I instruct", pressed into service as a noun. A praeceptum is a rule handed down by someone with the standing to hand it down: a teacher, a father, a bishop, a god. That is why the praeceptor is the schoolmaster.

Two practical notes. The word is close in sense to mandatum, but a mandatum is a specific order to a specific person while a praeceptum is a standing rule. And this corpus spells it both ways: medieval scribes flattened ae to e, so precepto and praecepto are the same word in the same text, sometimes on the same page. Read past the spelling.