Prefixes and Assimilation

Prefixes which cause trouble are the ones which change their final letter according to the first letter of the base they are attached to. This change is called assimilation.

Here are some examples which may be helpful: [the prefix changes in the following ways]

ad-   to, toward     
a-      averse, ascribe
ac-       accept, acclaim
af-      affect, affix
ag-      aggrandize, aggregate
al-      allude, allot
an-   annex, announce
ap-      appendage, applaud
ar-      arrive, arrange
as-      assemble, assign
at-      attest, attendance

con      together, with, or   very = indicating an *intensification* of whatever the base is]
co-      coefficient, coagul ate  
cog-      cogent, cognate
col-      collusion, colloquium
cor-      correlate, corrode

ex-   out, from, away
e-    evoke, eliminate
ef-       efficacious, effect

in-   in, into, on[used with verbs and nouns]
il-      illuminate, illiterate
im-      import, imbibe
ir-      irrigate, irradiate  

in-   not [used with adjectives]
ig-      ignoble, ignorant
il-      illicit, illegal
im -     immodest, improper
ir-      irrational, irregular  

ob-      toward, against
oc-      occult, occur
of-   offend
op-   oppose

sub-  under, up from under, somewhat  
suc-      succinct, succumb
suf-      suffer, suffix
sug-      suggest
sup-      suppose, supply
sus-      suspend, sustain

This material was prepared by Prof. Virginia Hellenga for her students in CLAS225 Scientific Terminology at Monmouth College in Illinois. If you have any questions, you can contact her at