String Constants:

<string>          ::= '<characters>'
                    | ''

<characters>      ::= <character>
                    | <characters> <character>

<character>       ::= ''
                    | {any character other than ' } 
Character string constants, and bit constants longer than 32 bits, are stored as string constants. A string constant is represented by a descriptor, a fixed value containing the length of the string in the high-order byte and the address of the string in the low-order three bytes. The maximum length of a string constant is 256 characters, or 2048 bits. The null string represented by '' is a string of zero length. Two successive single quotes within a character string constant are used to represent one single quote.

The following are valid character strings:

  'I don''t think so.'