Percussion reduction, or percussion flaking, refers to removal of flakes by impact. Generally, a core or other objective piece, such as a partially formed tool, is held in one hand, and struck with a hammer or percussor. Alternatively, the objective piece can also be struck between a stationary anvil-stone, known as bipolar percussion. Percussion can also be done by throwing the objective piece at an anvil stone. This is sometimes called projectile percussion. Percussors are traditionally either a stone cobble or pebble, often referred to as a hammerstone, or a billet made of bone, antler, or wood. Often, flakes are struck from a core using a punch, in which case the percussor never actually makes contact with the objective piece. This technique is referred to as indirect percussion.