In real world, we meet many constants, such as pi = 3.1415926513...
Pascal has a good facility to use the constants.

How to define our own constant ?
 Just like a variable definition, we just give const above our constants AND start tayloring our needs.

const

  :

  myconst = 1234;

  :

  :

  (Like variable definition)

We just omit the constant type, Pascal will understand it. We can use it just like a normal variable. Example :

const

  myconst = 1234;

var

  i : word;

begin

  i:=40;

  writeln(i*myconst);

end.

Constants is unchangeable. If you try to change it, Pascal will say error.

const

  myconst : mytype = thevalue;

example:

const

  abc : word = 1234;

You may not declare constants with values exceeding the type, like these :

const

  aaa : word = 100000;  { Illegal because word is integer ranging 0-65535 }

  bbb : byte = 123.44;  { Illegal because of the fractional part }

  ccc : string = 123;   { Illegal because incompatible type }

How can we modify them inside our program ?

const

  myconst : word = 1234;

var

  i : word;

begin

  i:=40; myconst:=1000;

  writeln(i*myconst);

end.