# X vs xi

> English confusable word pair

## The two words

### X
*num · IPA /ɛks/ · frequency rank #647*
An unknown quantity or unknown value.

### xi
*noun · IPA /zaɪ/ · frequency rank #5,928*
The 14th letter of Classical and Modern Greek. The 15th in Ancient and Old Greek.

## Source
Compiled from Wiktionary via kaikki.org (CC BY-SA). Data vintage: 2026-05-06.
Canonical page: https://plainspell.com/en/vs/x-vs-xi