A finger is a protruding cell in an oscillator or dying spark, with the ability to modify a nearby active reaction. Like a thumb, a finger cell appears at the edge of a reaction envelope and is the only live cell in its row or column. The finger spark remains alive for two ticks before dying, whereas a thumb cell dies after one tick. Because the key cell is kept alive for an extra tick, an alternate technical term is "held (orthogonal) bit spark". A "held diagonal bit spark" is not possible in B3/S23 for obvious reasons.