These are arrays of objects that follow sorting rules, i.e. objects are added or removed such that the sort order is maintained. A sort order may be alphabetical, numerical, etc. In a sorted array, finding an element is O(Log N), since a binary "divide and conquer" algorithm can be used; i.e., the program begins at the middle of the list, throws out half of it, then looks at the remaining half, throws out half again, etc. Removal involves a "crunch" algorithm that prevents gaps in the sequence.