Graded (or gradable) antonyms are word pairs whose meanings are opposite and which lie on a continuous spectrum (hot, cold).
Relational antonyms are word pairs where opposite makes sense only in the context of the relationship between the two meanings (teacher, pupil).
Some words lack a lexical opposite due to an accidental gap in the language's lexicon.
For example, "undevout" is found in Webster's 1828 dictionary, while the prefix pattern of "non-person" could theoretically extend to "non-platypus."
Other examples include: mortal : immortal, exit : entrance, exhale : inhale, occupied : vacant.
A relational antonym is one of a pair of words that refer to a relationship from opposite points of view.
Other examples include: husband : wife, doctor : patient, predator : prey, teach : learn, servant : master, come : go, parent : child.