Music is the production of rhythmically ordered sound. Music can be produced by means of the human voice or using musical instruments. Musical instruments vary from two sticks knocked together to produce a rhythm, to electronic synthesizers that can produce any sound imaginable (and a few that aren't). Almost every society has produced music, and the earliest music was often used to accompany dance, which consists of rhythmic movements of the body. The music that ordinary people learn, compose and pass on by word of mouth is sometimes referred to as folk music. Many societies have also developed a classical music tradition, a form of music that follows certain rules of composition and that is either recorded in written form or passed on from master to student in a formal process of education. Europe, Japan, India, Iran, Mali, Indonesia and many other countries have developed sophisticated traditions of classical music.