Canons can also be described according to the intervals between two voices. If one voice starts on a C and the next voice starts the same tune on an F above this is a “canon at the fourth” (because the interval (distance) from C to F is called a “perfect fourth”). If the second voice has the tune upside down (inversion) this is called “canon in inversion”. If the second voice has the tune at half the speed (each note being twice as long) this is a “canon in augmentation” or an “augmented canon”. The opposite is a “canon in diminution”.