Chi shares in the Chi-Rho monogram discussed under rho elsewhere in this atlas, formed by overlaying chi's crossed strokes on rho's single stroke to abbreviate the Greek Christos, Christ; the same abbreviating logic survives independently in the English word Xmas, which uses the Latin letter X to stand in for chi and, by extension, for Christ, a usage documented in English at least since the 16th century and often mistaken today for a purely secular shortening rather than a centuries-old scribal abbreviation.
In modern statistics, chi-squared, written with a superscript two after the letter, denotes one of the most widely used tests in the field, developed chiefly by Karl Pearson around 1900 to measure how far observed data departs from an expected distribution, a tool now standard across the sciences and social sciences alike. Chi also appears in physics as a symbol for electric susceptibility and other technical coefficients. None of these uses, ancient or modern, draws on chi's own classical Greek sound value or its place in the Milesian numeral system, both covered elsewhere in this atlas; the letter's afterlife runs almost entirely through its graphic resemblance to a cross, X, rather than through its spoken value.