The Greek letter chi, represented by Χ in uppercase and χ in lowercase, starts its story in the ancient Semitic world. Early Greek writers borrowed a symbol from Phoenician writing that may have carried a meaning like "support." Nobody knows exactly which Phoenician word sparked its use, but the cross-like design made it stand out. Greek scribes needed a letter for a sound that differed from kappa, so they adapted an X shape that proved both striking and practical.

In its early days, chi denoted an aspirated sound, one that began as a hard, breathy consonant. As time passed, pronunciations shifted. In classical settings, chi sounded much like a hard “k” with an extra puff of air. Over the centuries, the visual form of chi remained simple. Even as calligraphers and stone-carvers in ancient and medieval Greece added personal touches, the letter kept its clear-cut shape.

Modern use of chi reaches far beyond the world of language. Mathematicians depend on it to represent the chi-squared statistic in tests that check how well observed data fit expected patterns. Scientists use chi to denote variables in equations across different fields, and programmers sometimes adopt it when naming functions or constants. Early Christians also embraced chi, merging it with another letter to create the Chi-Rho symbol, an early identifier for a central religious figure.