Musical tuning
Just intonation, in which the ratios of the frequencies between all notes are based on whole numbers with relatively low prime factors, such as 3:2, 5:4, 7:4, or 64:45; or in which all pitches are based on the harmonic series, which are all whole number multiples of a single tone. Such a system can be used on instruments such as lutes, but not on keyboard instruments.