There are three main methods of classifying musical instruments: the Hornbostel-Sachs system, Western classification, and Chinese classification. The Hornbostel-Sachs system divides instruments into four main groups based on how they produce sound: idiophones, membranophones, chordophones, and aerophones. Western classification separates instruments into woodwinds, strings, brass, percussion, and keyboard. The oldest known scheme is Chinese classification from 3000 BC, which groups instruments based on the material they are made of such as stone, wood, silk, or bamboo.