The semantron (Greek: σήμαντρον) or semanterion (σημαντήριον); also called a xylon (ξύλον) (Romanian: toacă; Russian: било, bilo; Bulgarian: клепало, klepalo) is a percussion instrument made of a long, well-planed piece of timber, usually heart of maple (but also beech), from 12 feet and upwards in length, by 1½ feet broad, and 9 inches in thickness. It is used chiefly in monasteries to summon the brethren to prayer, or to lead processions.