Unicode assigns each character a combining class, which is identified by a numerical value. Non-combining characters have class number 0, while combining characters have a positive combining class value. To obtain the canonical ordering, every substring of characters having non-zero combining class value must be sorted by the combining class value using a stable sorting algorithm. Stable sorting is required because combining characters with the same class value are assumed to interact typographically, thus the two possible orders are "not" considered equivalent.