lex

Lex, legis is third declension feminine, a consonant stem, so the genitive plural is legum. As with rex and vox, the nominative x is c or g plus s and hides the stem leg-.

Latin keeps three legal words apart and it matters. A lex is a specific statute, something proposed, voted and recorded, with a name attached: lex Iulia, lex Cornelia. Ius is right and law as a system, the abstract entitlement. Mos is custom carrying the force of law. The reading trap is homography: legis is also "you read" and legi is also "I read" or "to be gathered", from lego; lege is both the ablative of lex and the imperative of lego. The context does all the work. Family: legalis, legitimus, collega, privilegium. English legal, legislate, legitimate, loyal; French loi, Spanish ley.