Esperanto does not use the letter x. When roots from other languages are adopted into Esperanto, the letter x gets converted to "ks" or "kz".
However, there is a common convention, where Esperanto's accented letters (ĉ, ĝ, ĥ, ĵ, ŝ, ŭ) cannot be typed, of signifying them with a following x (cx, gx, hx, jx, sx, ux). This is referred to as x-sistemo, x-kodo, or iksokodo.
- iksokodo (“x-code (see below)”)