The monochord consists of a metal string stretched over a hollow resonating body. Using a movable bridge the string can be divided into two portions whose lengths may be set at any ratio to give various pitches and musical intervals when plucked. According to tradition, the ancient Greek philosopher Pythagoras (fl. 4th century BC) discovered the correspondence between simple ratios of string length and consonant musical intervals. Dividing the string into sections in the ratios 2:1 or 3:2, for example, yields the intervals of a perfect octave and perfect fifth respectively.
Pythagoras and his followers believed that the whole universe could be understood in terms of musical harmonies and simple mathematical ratios. Until at least the 17th century astronomers such as Johannes Kepler (1571-1630) believed that the planets moved in such a way as to produce a kind of cosmic harmony. Astronomy and the study of musical harmony thus went hand in hand and music was considered a proper part of science throughout the medieval and into the early modern period.
