Ancient Egyptian and Greek philosophers used an early form of trigonometry that involved calculating chords to obtain the angles of a triangle. This method was effective for Euclidean plane geometry, but the heart of trigonometry, the sine, was developed in India in the sixth century.
The first exposition of modern trigonometry was in a series of five books published by the Indian mathematician Vrahamihira. In this work, Vrahamihira relied on an older Babylonian table of chords to develop his own table of sines. This work was adopted by Islamic mathematicians and eventually passed to Europe, where the modern system of notation was developed by Leonhard Euler in the 18th century.