The way in which the intervals between notes on keyboard instruments are distributed throughout the scale to ensure that music in all keys sounds in tune. The problem can be illustrated by a piano keyboard. Taking a low C and a high C seven octaves above, the interval should be 2⁷=128. However, in passing through the cycle of twelve keys, each using as its fundamental the fifth of its predecessor, the interval between Cs becomes (3/2)¹²=129.75. The difference between 129.75 and 128 is known as the comma of Pythagoras. The equal-temperament scale, which has been in use since the time of J. S. Bach, distributes the comma of Pythagoras equally between the twelve intervals of the scale over seven octaves. Thus each fifth becomes (128)^1/12=1.4983. All forms of temperament involve a measure of compromise; this system is now regarded as the best.