caput

Caput, capitis is third declension and NEUTER, which matters in practice: the nominative and the accusative are both caput, the plural is capita, and the genitive capitis gives you the stem capit- for everything else.

The word travels a long way from anatomy. It is the top or source of anything: caput fluminis is a river's source, caput viae the start of a road, caput libri a chapter, caput rei the main point. In law caput is a person's civil existence, which is why capitis deminutio is loss of legal status and why a charge that threatens it is a capital one. Counting people by heads is Roman too, in capita.

The stem gives capital, capitulate, chapter (through capitulum), decapitate, and through Vulgar Latin the French chef and English chief. Its derivative capitale, "property held as capital", is behind both cattle and chattel, which were once the same word and were once counted by the head.