The semantron or semandron (Greek: σήμαντρον), or semanterion (σημαντήριον), also called a xylon (ξύλον) (Romanian: toacă; Russian: било, bilo; Bulgarian, Macedonian, Serbian: клепало, klepalo; Arabic: ناقوس, nāqūs) is a percussion instrument used in monasteries to summon the monastics to prayer or at the start of a procession.

A Russian monk playing a semantron