Some keywords are reserved in ANSI C.This keyword we can use as such they have given.Before using those keywords keep one thing in mind the are lowercase.Some main ANSI C reserved are given below: asm: Begins assembly code and is not part of the ANSI standard. FORTRAN: The entry follows FORTRAN calling conventions; FORTRAN may be in lowercase for some implementations and is not part of the ANSI standard. PASCAL: The entry follows PASCAL calling conventions; PASCAL may be in lowercase for some implementations and is not part of the ANSI standard. Generally, the PASCAL conventions are identical to FORTRAN�s. const: The variable will be used as a constant and will not be modified. volatile: The compiler may make no assumptions about whether the variable�s value is current. This keyword limits optimization, and possibly slows program execution. signed: The variable is a signed integer (with the actual size unspecified). auto: The variable is created when the function is called, and is discarded when the function exits. An auto variable is not initialized by the compiler. continue: Passes control to the next iteration of a do(), for(), or while() statement. enum: An integer defining a range of values. The actual internal representation of the value is not significant. extern: The object is defined in a different source file.