There are two prefixes spelled in-. Both come from Latin, but they are not related to each other. The basic meaning of one prefix is “not.” Thus inactive means “not active.” This in- is related to and sometimes confused with the prefix un- that means “not” (unfortunately, there are also two un-’s). In fact, sometimes in- is used interchangeably with un-, as when incommunicative is used instead of uncommunicative. Before the consonants l and r, in- becomes il- and ir- respectively: illogical, irregular. Before the consonants b, m, and p, in- becomes im-: imbalanced, immeasurable, impossible.
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The second in- has for its basic meaning “in, within, or into.” For example, inlay means “to set something in something else.” In- is also a form of the prefix en-. And in pairs such as enclose/inclose, enquire/inquire, ensure/insure, the two prefixes can be used somewhat interchangeably. As with the other prefix in-, before the consonants l and r, in- becomes il- and ir-: illuminate, irrigate. Before the consonants b, m, and p, in- becomes im-: imbibe, immigrate, implant.
