The CHAR data type consists of single character values from  the
  ASCII  character  set.   The  largest possible value of the CHAR
  data type is the predefined constant MAXCHAR.

  To specify a  character  constant,  enclose  a  printable  ASCII
  character   in   single   quotation   marks.    To  specify  the
  single-quote character, enclose two single  quotation  marks  in
  single  quotation  marks.   Each  of  the  following  is a valid
  character constant:

     'A'
     '0'     {This is character 0, not the integer value 0}
     ''''    {The apostrophe character}
     '?'

  You   can   specify   nonprinting   characters,   such   as    a
  control-character,   by   writing   an   empty  string  followed
  immediately by the ordinal value of the character in  the  ASCII
  character  set,  or  by  using  the CHR function followed by the
  ordinal value of the character in the ASCII character set.   For
  example,   both  of  the  following  specify  the  bell  control
  character:

     ''(7)
     CHR(7)