Usually, the time required for a pulse to increase from 10% of its final value to 90% of its final value. Rise time is less frequently measured between the 5% and 95% points or the 1% and 99% points. Rise time is used to specify the transient response of an instrument, and is similar to its time constant, relaxation time, or response time, although these latter terms use (1 − e⁻¹), or about 63%, as the fractional change in state over which time is measured, beginning at an initial value. For example, if a step increase of 10° is applied to a thermometer registering 0° with a rise time of 50 s, the thermometer would increase from 1° to 9° in 50 s. Note that nothing is specified concerning the time required for the instrument to respond to the first or last 10%. See also time lag.