A plaindrome is a number whose hexadecimal digits are in nondecreasing order. The first few are 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 number that is not a plaindrome is called a katadrome.
The following table summarizes related classes of numbers.
| name | base-16 digit order |
| katadrome | strict descending |
| metadrome | strict ascending |
| nialpdrome | nonincreasing |
| plaindrome | nondecreasing |
