Prefix: in- (and its variants il-, im-, ir-)
Source: DfE, English Appendix 1: Spelling (2013) — Years 3–4 statutory prefix
Origin: Latin in-, meaning "not" or "into/within"
Purpose: Guide to the prefix in-/il-/im-/ir- — meaning 'not', assimilation rules, word list with definitions, and morpheme-based practice activities.
What it means
The prefix in- means not when used as a negation prefix:
incorrect = not correct
inactive = not active
invisible = not visible
It can also mean into or within in some words, but the negation meaning is what Appendix 1 teaches at Y3/4.
The assimilation rule — why in- changes form
When in- is added to a word beginning with certain letters, the n changes to match the first letter of the base word. This makes the word easier to pronounce. The process is called assimilation (the prefix assimilates to the base word).
| Base starts with | Prefix becomes | Example |
|---|---|---|
| l | il- | in- + legal → illegal |
| m or p | im- | in- + possible → impossible |
| r | ir- | in- + regular → irregular |
| Everything else | in- | in- + correct → incorrect |
Why does assimilation happen?
Try saying inlegal — the n and l clash awkwardly. Your mouth naturally shifts the n to l to make illegal, which flows more smoothly. Impossible is easier to say than inpossible. Irregular is easier than inregular.
This is not an exception or an irregularity — it is the regular rule working to make the language more natural to speak. The prefix is always in- in meaning; only its spelling changes.
in- words (base starts with most consonants or a vowel)
| Word | Meaning |
|---|---|
| incorrect | not correct |
| inactive | not active |
| incomplete | not complete |
| independent | not dependent |
| invisible | not visible |
| insecure | not secure |
| informal | not formal |
| indirect | not direct |
| injustice | lack of justice |
| inhuman | not human; cruel |
| inadequate | not adequate |
| inaccurate | not accurate |
| infinite | not finite; endless |
| innocent | not guilty (in- + nocent — not causing harm) |
il- words (base starts with l)
| Word | Meaning |
|---|---|
| illegal | not legal |
| illegible | not legible (can't be read) |
| illiterate | not literate (can't read or write) |
| illogical | not logical |
| illegitimate | not legitimate; unlawful |
im- words (base starts with m or p)
| Word | Meaning |
|---|---|
| impossible | not possible |
| immature | not mature |
| immortal | not mortal; living forever |
| immovable | not movable |
| imperfect | not perfect |
| impatient | not patient |
| impractical | not practical |
| immense | not small; enormous |
| immediate | not with a gap; instant |
| immobile | not able to move |
ir- words (base starts with r)
| Word | Meaning |
|---|---|
| irregular | not regular |
| irrelevant | not relevant |
| irresponsible | not responsible |
| irreversible | not reversible |
| irrational | not rational |
| irresistible | not resistible (impossible to resist) |
| irreplaceable | not replaceable |
Double letters explained
The double letters in illegal, immature, irregular are NOT from the doubling rule — they come from assimilation:
- il- + legal → illegal (the l from il- + the l from legal)
- im- + mature → immature (the m from im- + the m from mature)
- ir- + regular → irregular (the r from ir- + the r from regular)
The double letters are a visual signal of assimilation: you can see the prefix and the base word clearly, each contributing one letter to the pair.
Common mistakes
| Wrong | Right | Reason |
|---|---|---|
| inlegal | illegal | in- + l → il- |
| inpossible | impossible | in- + p → im- |
| inregular | irregular | in- + r → ir- |
| illogical written as inlogical | illogical | assimilation to il- |
| iregular | irregular | double r — both from prefix and base |
Word sort
Identify which form of the prefix each word uses.
Words: incorrect · illegal · impossible · irregular · inactive · illegible · immature · irresponsible · incomplete · illiterate · imperfect · irreversible · invisible · illogical · impatient
| in- | il- | im- | ir- |
|---|---|---|---|
Classroom activity: Find the base
For each word, find the base word and check which in- variant is used. Ask: why that variant?
- illegal → legal → starts with l → il-
- impractical → practical → starts with p → im-
- irregular → regular → starts with r → ir-
- incomplete → complete → starts with c → in-
Dictation sentences
- It was illegal and irresponsible to leave the building incomplete.
- The irregular pattern made the design look imperfect and illogical.
- She remained invisible and inactive while the immature argument continued.
- The impossible task became irreversible once the incorrect code was entered.
- An illegible note left by an irresponsible patient made the whole record inaccurate.
Source: DfE English Appendix 1: Spelling (2013). Etymology: Latin in-, from Proto-Indo-European ne- (not). Assimilation is a standard phonological process documented in all Latin grammars.