However, derivational affixes do not necessarily alter the lexical category; they may change merely the meaning of the base and leave the category unchanged. A prefix ("write" → " re-write"; "lord" → "over-lord") rarely changes the lexical category in English. The prefix "un-" applies to adjectives ("healthy" → "unhealthy") and some verbs ("do" → "undo") but rarely to nouns. A few exceptions are the derivational prefixes "en-" and "be-". "En-" (replaced by "em-" before labials) is usually a transitive marker on verbs, but it can also be applied to adjectives and nouns to form transitive verbs: "circle" (verb) → "encircle" (verb) but "rich" (adj) → "enrich" (verb), "large" (adj) → "enlarge" (verb), "rapture" (noun) → "enrapture" (verb), "slave" (noun) → "enslave" (verb).