The LaTeX code for the Greek uppercase letter Chi (Χ) is \Chi. However, Greek uppercase Chi looks identical to the Latin letter X, which is why there's rarely a specific command for the uppercase, and you can use the regular Latin X in most cases. For the lowercase chi (χ), use \chi.

• $ \Chi $ displays $\Chi$
• $ \chi $ displays $\chi$

Here’s how to use them in a LaTeX document:

\documentclass{article}
\begin{document}

The uppercase Greek letter Chi: X or \Chi

The lowercase Greek letter chi: $\chi$

\end{document}

Make sure you are in math mode (using $...$ or $$ ... $$) when writing Greek letters other than the uppercase Chi that looks like the Latin X.