Why is the ninth month called September?
Latin, literally "by the head," from per (see per) + capita "head" (see capital).
"head," in various senses, from Latin caput (see capitulum).
caput cap·ut (kāp'ut, -ət)n. pl. cap·i·ta (kāp'ĭ-tə) The head.
A Latin phrase literally meaning “by heads,” and translated as “for each person.” It is a common unit for expressing data in statistics. A country's per capita personal income, for example, is the average personal income per person.