A list is an ordered sequence of elements, and each element has a unique index. On the AP CSP reference sheet, indexes run from 1 to the length of the list, and any index outside that range is an error. A list lets many related values be treated as one value, a data abstraction that manages complexity. Traversing a list visits its elements in order, which is how programs find a minimum or maximum, a sum or average, or search for a value one element at a time.