You can open Keyboard Editor by clicking Edit in the Keyboard Settings dialog box.

Keyboard Editor Main Window

The main window shows a keyboard layout graphically and mappings for each key as a list. Keyboard Editor supports two layout modes: Normal Layout and Compose Layout. If you click the Keyboard Edit Mode button on the toolbar, the layout is toggled.

Normal Layout shows basic keys on the keyboard and you can edit the keysyms for each key. A keysym is a symbolic name assigned to a key.

Compose Layout shows the compose keys as highlighted and you can edit compose sequences for dead keys in European keyboards.