Tetrad

The story of the word

The Pythagoreans swore oaths by the τετρακτύς, ten pebbles laid in four rows to make a triangle, in which they claimed to read the whole of music and number. τετράς is the plain word beside it, a group of four, and English took it in the seventeenth century as a mathematician's term. Biology later borrowed it for the four chromatids of a paired chromosome in meiosis, and chemistry for a four-valent group.

How it travelled

Ancient Greek → Late Latin → English

First recorded

17th century