 This distinction gives rise to the notion of a quotient set.Similarly, the sets

are not equal sets — the first consists of letters, while the second consists of numbers — but they are both sets of three elements and thus isomorphic, meaning that there is a bijection between them.For example

However, there are other choices of isomorphism, such as

and these sets cannot be identified without making such a choice — any statement that identifies them "depends on choice of identification".