The difference is that **STABLE-SORT** is guaranteed to not reorder any elements considered equivalent by the predicate while **SORT** guarantees only that the result is sorted and may reorder equivalent elements.

Typically you won’t care about the unsorted version of a sequence after you’ve sorted it, so it makes sense to allow and **STABLE-SORT** to destroy the sequence in the course of sorting it. But it does mean you need to remember to write the following:10

Both these functions also take a keyword argument, :key, which, like the :key argument in other sequence functions, should be a function and will be used to extract the values to be passed to the sorting predicate in the place of the actual elements. The extracted keys are used only to determine the ordering of elements; the sequence returned will contain the actual elements of the argument sequence.