Incremental rotary encoders or rotary pulse generators are multi-turn sensors that use optical, mechanical, or magnetic index-counting for angular measurement. They do not contain an absolute reference. Instead, incremental rotary encoders count relative to the turn-on position. The number of successive cycles (signals) corresponds to the resolvable mechanical increments of motion. The signal provides logic states "0" and "1" alternately for each successive cycle of resolution.  For example, optical encoders use a transmitter-receiver to count the opaque lines and thus the angular increment. Multiple transmitter-receiver sets may be arranged to provide multiple counts per line. One common technique is to offset two sets a half line-width apart. This results in four counts per line. This technique of enhancing resolution via out-of-phase signals is known as quadrature.