A scale of measurement which can be used to measure the difference, or distance, between two general states or points. For example, use of a ruler to measure length, use of a stop-watch to measure a time interval, or measurement of a musical interval (octave, fifth, etc.).

In a ratio scale the difference, or distance, is measured between a state or point of interest and a standard state or point. For example, height above sea-level, distance from London, frequency of a musical note (in cycles per second), temperature in °Kelvin (above absolute zero), clock-time (13:05 on 5 November 2001 ad). Confusingly, comparison of ratio scale measurements gives an interval scale, and in music an interval is measured by the ratio of the frequencies.