| 1 | Irish | -ach | Forms nouns from other nouns and adjectives with... |
| 2 | Middle Irish | -ach | Forms adjectives meaning "related to, having,... |
| 3 | Old Irish | -ach | Forms adjectives meaning "related to, having,... |
| 4 | Proto-Celtic | -ākos | Forms adjectives from nouns and adjectives with... |
| 5 | Proto-Indo-European | -eh₂ḱos | — |
| 6 | Proto-Indo-European | -kos | Creates deadjectival and desubstantival... |