On some non-U.S. English keyboards, certain keys are defined to add a diacritic to a letter. These are called “dead keys” because they don’t generate characters by themselves. For instance, when a German keyboard is installed, the key that is in the same position as the +/= key on a U.S. keyboard is a dead key for the grave accent (`) when shifted and the acute accent (´) when unshifted.

When a user presses this dead key, your window procedure receives a WM_DEADCHAR message with wParam equal to ASCII or Unicode code for the diacritic by itself. When the user then presses a letter key that can be written with this diacritic (for instance, the A key), the window procedure receives a WM_CHAR message where wParam is the ANSI code for the letter `a’ with the diacritic.

Thus, your program does not have to process the WM_DEADCHAR message because the WM_CHAR message gives the program all the information it needs. The Windows logic even has built-in error handling: If the dead key is followed by a letter that can’t take a diacritic (such as `s’), the window procedure receives two WM_CHAR messages in a row—the first with wParam equal to the ASCII code for the diacritic by itself (the same wParam value delivered with the WM_DEADCHAR message) and the second with wParam equal to the ASCII code for the letter `s’.

Of course, the best way to get a feel for this is to see it in action. You need to load a foreign keyboard that uses dead keys, such as the German keyboard that I described earlier. You do this in the Control Panel by selecting Keyboard and then the Language tab. Then you need an application that shows you the details of every keyboard message a program can receive.