Keysyms

Keysyms, short for key symbols, are 32-bit identifiers used by the X Window System to represent the symbolic value of a key in keyboard input events. They sit between the hardware KeyCode (the physical key) and characters or actions, and are produced by the keyboard mapping managed by XKB. Applications read KeySyms from events or translate them to Unicode or other representations, often by comparing them with predefined constants such as XK_A, XK_Return, or XK_Left.

Keysyms cover both printable characters and a wide range of special keys. Printable ASCII characters typically map directly to their Unicode code points (for example the KeySym for 'A' corresponds to 0x0041). Special keys, including function keys, arrows, and modifiers, have values in the 0xFF00 to 0xFFFF range (for example XK_Return, XK_Left, XK_F1). A single physical key may yield multiple KeySyms depending on the keyboard state and layout (levels and modifiers).

Keysyms are defined in the X11 families of headers (notably keysymdef.h) and are part of the X11 protocol and XKB mapping process. The relationship to Unicode is provided by the XKB server and libraries, but not every KeySym has a direct Unicode equivalent. In practice, software libraries such as Xlib and XCB use KeySyms to interpret input, while toolkits may provide higher-level abstractions for text input and command shortcuts. While newer input systems exist in other environments, Keysyms remain a core concept for X11-based applications and interoperability.