In mathematics, a rate is the ratio between two related quantities. Often it is a rate of change. If the unit or quantity in respect of which something is changing is not specified, usually the rate is per unit time. However, a rate of change can be specified per unit time, or per unit of length or mass or another quantity. The most common type of rate is "per unit time", such as speed, heart rate and flux. Ratios that have a non-time denominator include exchange rates, literacy rates and electric flux.