The characters with case are a subset of the alphabetic[1] characters. A character with case has the property of being either uppercase or lowercase. Every character with case is in one-to-one correspondence with some other character with the opposite case.
13.1.4.3.1 Uppercase Characters
13.1.4.3.2 Lowercase Characters
13.1.4.3.3 Corresponding Characters in the Other Case
13.1.4.3.4 Case of Implementation-Defined Characters