Which to use
“iff” is a conjunction and “is” is a verb - they look or sound alike but fill different roles in a sentence.
- #45,443
- “iff” frequency rank
- #8
- “is” frequency rank
- 45451
- confusion score
Side-by-Side Comparison
| Feature | iff | is |
|---|---|---|
| Definition | Abbreviation of if and only if. | third-person singular simple present indicative of be |
Where the eye mixes iff and is
Muted = shared; highlighted = the visual trap. They share 1 letter in sequence. iff runs 3 letters (VCC) against is's 2 (VC) - the length gap alone is often enough to separate them once you know to check it.
Visual trap between iff and is
These two trip readers on the page rather than in the ear: iff is /ɪf ən(d)ˈəʊn.li ɪf/ while is is /ɪz/. Spoken aloud the problem disappears. In writing it persists, because they differ by 1 letter(s) in length, and the parts of speech differ too (conjunction vs verb), which is usually the fastest check. Our composite confusion score for this pair is 45451, 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/. is is at rank #8, tagged as averb, pronounced /ɪz/.
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 45451, this pair ranks #221,157 of 530,003 scored English confusable pairs - roughly mid-pack for confusability.
Orthographic DNA of iff vs is
Shared letters: i. Private to "iff": f. Private to "is": s.
"iff" · 3 letters · shape VCC · "is" · 2 letters · shape VC
Frequently Asked Questions
Can "iff" and "is" be used interchangeably?
In ordinary English text, does "iff" or "is" show up more?
What to do next
Visual near-miss handled. Browse the word pages or a neighbour when the next look-alike shows up.
- Full dictionary entry for "iff" with definition, IPA, and generated misspellings. Word entry
- Full dictionary entry for "is" - compare definitions side by side with the headword page. Word entry
- This pair ranks #221,157 of 530,003 scored English confusable pairs in this corpus vintage. Corpus statistics
- "iff" and "is" also get mixed up with nearby pairs, the highest-scored neighbour is "iff vs ith". Nearby pair
Confusion rank #221,157 of 530,003 scored English pairs, corpus vintage 2026-09-03, not spelling advice.