Which to use
“iff” is a conjunction and “ill” is an adjective - they look or sound alike but fill different roles in a sentence.
- #45,443
- “iff” frequency rank
- #1,946
- “ill” frequency rank
- 47389
- confusion score
Side-by-Side Comparison
| Feature | iff | ill |
|---|---|---|
| Definition | Abbreviation of if and only if. | Evil; wicked (of people). |
Where the spellings diverge
Shared letters are muted; the letters that actually set iff and ill apart are highlighted. They share 1 letter in sequence, which is exactly why the eye skips the difference.
Spelling & Dictionary Insight
These two trip readers on the page rather than in the ear: iff is /ɪf ən(d)ˈəʊn.li ɪf/ while ill is /ɪl/. Spoken aloud the problem disappears. In writing it persists, because they share most of their letters but differ in 2 positions, and the parts of speech differ too (conjunction vs adjective), which is usually the fastest check. Our composite confusion score for this pair is 47389, derived from the frequency rank of both members and their visual similarity.
iff is recorded at frequency rank #45,443, classified as aconj, pronounced /ɪf ən(d)ˈəʊn.li ɪf/. ill is at rank #1,946, tagged as anadj, pronounced /ɪl/.
Glosses for this pair are partially populated in our dataset, but the full side-by-side definitions above should still guide you to the right choice.
With a confusion score of 47389, this pair ranks #202,913 of 530,003 scored English confusable pairs - roughly mid-pack for confusability.
Orthographic DNA of iff vs ill
Shared letters: i. Private to "iff": f. Private to "ill": l.
"iff" · 3 letters · shape VCC · "ill" · 3 letters · shape VCC
Frequency comparison
Frequently Asked Questions
Can "iff" and "ill" be used interchangeably?
Which is more common, "iff" or "ill"?
Remembering iff vs ill
The fastest way to pick the right one every time.
- Check the role first: if you need a conjunction, it's “iff”; for an adjective, it's “ill”.
- See each word in full, definition, IPA, etymology and its other confusables. Full “iff” entry
- Browse more pairs most likely to be confused. Most confusable