Mathematical Bold Capital X

It marks a mathematical X whose role the author defines. In one equation, plain and bold letters can name different variables. A bold capital X may stand for a vector, a matrix, or another object; its weight alone does not tell you which.

The exact bold X is not a newly invented sign; it applies bold type to the Latin letter X. A documented early mathematical convention appears in J. Willard Gibbs’s 1881 vector analysis and E. B. Wilson’s influential 1901 textbook: Gibbs used Greek lowercase for vectors and English lowercase for scalars; Wilson used Clarendon, a bold typeface, for vectors and ordinary type for scalars. Hermann Weyl’s 1921 book used bold x for a vector and plain indexed xᵢ for a component. Later authors assigned the styles to other distinctions, so the letter has no universal mathematical meaning.

The Unicode chart’s drawing is a sample, not a required design; mathematical fonts may give 𝐗 different exact shapes.

How do I type a bold X in LaTeX?

In LaTeX math, \mathbf{X} typesets an upright bold X. It styles the letter inside a formula instead of inserting the standalone Unicode character 𝐗.