An in-place sort algorithm that repeatedly reorders different, small subsets of the input until the entire array is ordered. On each pass it handles i sets of n/i items, where n is the total number of items. Each set is every i^th item, e.g. set 1 is item 1, 1+i, 1+2i, etc., set 2 is item 2, 2+i, etc. On each succeeding pass, the increment or gap, i, is reduced until it is 1 for the last pass.