In an equal temperament, relative error is the amount by which the mapping of an interval or harmonic deviates from its just value. For example, in 12edo the relative error of 3/2 is -2.0%, and the relative error of 5/4 is +13.7%. We can plot the relative error of harmonics in a graph like the one on the right. This graph lets us calculate the relative error of intervals. For example, the relative error of 6/5 is 13.7%-(-2.0%)=15.6% (not 15.7% due to rounding error). However, one may not always want to use the nearest approximation of every harmonic. For example, using the second best approximation for harmonic 13, with relative error +59.5%, actually gives us less error overall due to cancellation of errors between harmonics.
