A plaindrome is a nonnegative integer whose hexadecimal digits are in nondecreasing order. The first few are 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 17, 18, 19, 20, 21, 22, 23, 24, ... (OEIS A023757).
A katadrome instead has hexadecimal digits in strictly decreasing order.
The following table summarizes related classes of numbers.
| name | base-16 digit order |
| katadrome | strict descending |
| metadrome | strict ascending |
| nialpdrome | nonincreasing |
| plaindrome | nondecreasing |
