A constant is a numeric or character value that  does  not  change  during
  program execution.  BASIC accepts floating-point, integer, packed decimal,
  and character string constants.  You can create a named  constant  of  any
  data  type  with the DECLARE statement.  You can also use explicit literal
  notation to specify the value and data type of a numeric literal.

1  –  Literal notation

  The format for explicit literal notation is:

      [radix] num-str-lit [data-type]

  Radix specifies an optional base and can be:

   o  D Decimal (base 10) - the default
   o  B Binary (base 2)
   o  O Octal (base 8)
   o  X Hexadecimal (base 16)
   o  A ASCII

  The numeric string can consist of digits and an optional decimal  point.
  You  can  also  use  E notation for floating-point constants.  A leading
  minus sign cannot appear inside the quotation marks.   Data  type  is  a
  single letter abbreviation a data type keyword:

   o  B BYTE
   o  W WORD
   o  L LONG
   o  Q QUAD
   o  F SINGLE
   o  D DOUBLE
   o  G GFLOAT
   o  S SFLOAT
   o  T TFLOAT
   o  X XFLOAT
   o  P DECIMAL

  Data type can also be C, which specifies a  single-character  string  in
  terms  of  its 8-bit ASCII value.  For the C data type, the value of the
  numeric string must be between 0 and 255, inclusive.  For example:

  B"11111111"B = BYTE binary constant (-1)
  X"FF"B       = BYTE hexadecimal constant (-1)
  O"377"B      = BYTE octal constant (-1)
  "7"C         = CHARACTER constant (ASCII decimal value 7)

2  –  Floating-point

  A floating-point constant is a literal or named  constant  with  one  or
  more  decimal  digits,  either positive or negative, an optional decimal
  point, and an exponent of 10 (E notation).  If the default data type  is
  INTEGER, a decimal point or an E is required or BASIC treats the literal
  as an integer.  If the default data type is DECIMAL, an E is required or
  BASIC treats the literal as a DECIMAL.

3  –  Integer

  An integer constant is a literal or named  constant  with  one  or  more
  digits,  either  positive  or negative, with no fractional digits and an
  optional trailing percent sign (%).  The percent sign  is  required  for
  integer literals if the default data type is not INTEGER.

4  –  Packed Decimal

  A packed decimal constant is a number, either positive or negative, that
  has  a  specified  number  of  digits  (d) and a specified decimal point
  position (s).  You can specify the d and s values when you  declare  the
  constant  or use explicit literal notation.  If you do not specify d and
  s values, BASIC uses the current defaults for DECIMAL.

5  –  String

  String constants are either  string  literals  or  named  constants.   A
  string  literal  is  a series of characters enclosed in double quotation
  marks ("text") or single quotation marks ('text').  You can embed double
  quotation  marks within single quotation marks ('"text"') and vice versa
  ("'text'").  Note, however, that  the  outer  quotation  marks  must  be
  paired.

6  –  Named

  To name a constant within a program, use the DECLARE CONSTANT statement.
  To  name  a  constant  external  to a program, use the EXTERNAL CONSTANT
  statement.