__UI_KeyHit

__UI_KeyHit is a global variable of type LONG INTEGER that holds the scan code of the last key pressed by the user. Positive values mean a key was pressed, negative values indicate a key was released.

This variable can be accessed anywhere, but it's usually manipulated in KeyPress event handlers, especially for Textbox controls, as it can be used to change the key presses that actually get sent to the control.