What's the meaning of #define WM_KEYFIRST 0x0100, #define WM_KEYDOWN 0x0100, #define WM_KEYUP 0x0101. Why 0x0100 is twice?

Question

What\'s the meaning of #define WM_KEYFIRST 0x0100, #define WM_KEYDOWN 0x0100, #define WM_KEYUP 0x0101? Those are lines 1038, 1039 and 1040 in winuser.h library. Why those value