The suffix -able , and its variants, is attached to Latin verb roots or English transitive verbs to form adjectives. (In a small set of cases, -able is attached to a noun, e.g. knowledgeable and marriageable.) For spelling purposes, it is useful to know that -able is more likely to be added to a whole English word (comparable, adorable ), while -ible is more likely to follow a bound root (visible , legible ). However,there are still many words which combine a root with -able (capable , inevitable ). The suffix -ity can be added to any adjective ending in -able , -ible , -ble to produce a corresponding noun ending in -ability, -ibility, -bility (possible + -ity > possibility; irrevocable + -ity > irrevocability.)