Say “in-possible” quickly, and your lips close for the p before you finish the n. Over centuries, speakers smoothed that bump, and the prefix became im-: impossible. Many prefixes that end in n, d, b, s or x change their last letter to match the next sound. It even has a name, assimilation, from ad- + simil (like): “making alike.”
Key idea
A doubled consonant after a short prefix (acc-, aff-, coll-, imm-, irr-, supp-) is often the sign of a prefix that changed its last letter. Undo the change to find the prefix: ac-cede is ad- + cede.
Prefixes in disguise
How the shapes change
| Prefix, before | Becomes | Words from the list |
|---|---|---|
| in- (not) before b, m, p | im- | impeccable, immutable, impartial |
| in- (not) before l | il- | illicit |
| in- (not) before r | ir- | irrevocable, irrefutable |
| con- (with) before b, m, p | com- | commend, compel, compunction |
| con- (with) before l / r | col- / cor- | colloquial, collusion / corroborate |
| ad- (to) before c, f, g, l | ac-, af-, ag-, al- | accede, affable, aggregate, alleviate |
| ad- (to) before n, p, r, s | an-, ap-, ar-, as- | annihilate, apprehensive, arrogant, assert |
| sub- (under) before c, p, r | suc-, sup-, sur- | succumb, suppress, surreptitious |
| ob- (against) before c, f, p | oc-, of-, op- | occur, offend, oppose (not on the list) |
| ex- (out) before f | ef- | effrontery |
| dis- (apart) before f | dif- | diffident, diffuse |
| syn- (with) before b, m, p / l | sym- / syl- | sympathy, symphony / syllable |
Worked example: Find the hidden prefix
What prefix is hiding in aggregate, and what does the word mean?
- The doubled gg after the first a is the clue: ad- became ag- before the g.
- What is left is greg (flock, as in congregate) + -ate.
- Put it together: “flocked toward one another,” many things gathered into one group.
Answer: ad- (to). An aggregate is a whole formed by combining many parts; a total.
Meet the word
Common mistake
Not every doubled letter marks a prefix: apple, arrow and attic have none. Check that what is left after the prefix is a real root, and that the meanings together fit the word.
Why it happens: the last sound of the prefix and the first sound of the root sit side by side, and speakers make them alike to say the word smoothly. Try saying con-lect and col-lect: the second is easier.