The organ is a keyboard instrument in which the sound is produced by air passing through pipes of various size and construction to give a wide variety of pitches and timbres. The instrument has its probable Western origin in the Hellenistic period, with the water-organ of Alexandria. Varying in size and mechanical efficiency, the organ had by the later 17th century given rise to an important school of performance, leading directly to the achievement of Johann Sebastian Bach in the first half of the 18th century. Technical developments have taken place since then, giving still greater versatility to the king of instruments.