Latin's shape-shifting "in-" prefix

Latin had a strict rule about not letting awkward consonant sounds bump into each other. The prefix "in-" means "not," and it started life attaching to every root the same way — but try saying "inlegible" out loud, fast. Your tongue wants to smear that "n" into the "l" that follows. Latin speakers gave in to that pull centuries ago, and the spelling followed the sound: "illegible," with a doubled "l" and no "n" in sight.

The rule

When the Latin prefix "in-" ("not") attaches to a root beginning with l, m, b, p, or r, its final "n" assimilates — changes to match the root's first letter — because Latin favored the easier-to-say blend over the original sound. Before l, "in-" becomes "il-" (in- + legible → illegible). Before m, b, or p, it becomes "im-" (in- + possible → impossible, in- + mobile → immobile). Before r, it becomes "ir-" (in- + regular → irregular). The unassimilated spelling — "inlegible," "inpossible," "inregular" — is never correct English, even though the "not" meaning is exactly the same; only the prefix's visible shape has changed to match its neighbor. Don't confuse this with the unrelated "un-" prefix (as in "unlawful" or "unpopular") — that one comes from Old English, not Latin, and it never assimilates no matter what letter follows.

Try it yourself