Only 3 basic data types are available in C: INTEGERS, CHARACTERS, and FLOATING-POINT (REAL) NUMBERS. These types may be further specified according to the size or accuracy desired. Both VARIABLES and CONSTANTS of these types may be defined.
The IDENTIFIER for a C variable - like that for a function - is any non-reserved sequence of letters (case is significant!), digits, and underscore (_) characters, with the restriction that the first character cannot be a digit and in general should not be an underscore. There is no restriction to the length of an identifier, although ANSI C compilers may ignore characters beyond the 31st. Commonly multi-word identifiers are described by using underscores (average_of_grades) to separate words or by beginning each new word with a capital letter (averageOfGrades). It is customary never to begin a variable identifier with a capital letter.