appear to be derived from a prefix suggesting opposition, yet the root term does not exist. An example is "inept," which seems to be "in-" + *"ept," although the word "ept" itself does not exist. Such words are known as unpaired words.Opposites may be viewed as a special type of incompatibility. Words that are incompatible create the following type of entailment (where X is a given word and Y is a different word incompatible with word X):

: sentence A is X