Pro's Defensive Front Terminology

I've have been looking at a 2004 Saints Offensive Playbook. I cannot fiqure out how the pros designate various defensive fronts such as a 57, 25 ,45, 47 ETC.. . Does anyone know how they arrive with these labels?
"57"
-----------------S------------M-----------------
-----------E--------T-----T--------E--W---------
-----------O---O---O---C---O---O--------------
-------------------------Q------------------------
--------------------------------F-----------------
-------------------------T------------------------

"25"
-----------------W-----------M----------------
-------------E----T-----T--------E-----S----
--------------O---O---C---O---O---O-----------
----------------------- Q------------------------
----------------F---------------------------------
------------------------T-----------------------

"45"
-------------------------M------W----------------
-----------S---E----T--------T-----E-----------
-----------O---O---O---C---O---O--------------
-------------------------Q------------------------
--------------------------------F-----------------
-------------------------T------------------------

"47-"
---------------------M-----------S--------------
----------W-E------T-----T--------E------------
--------------O---O---C---O---O---O-----------
----------------------- Q------------------------
----------------F---------------------------------
------------------------T-----------------------

"49"
---------------S--------M------W----------------
-----------E-------- T------- T----E-------------
-----------O---O---O---C---O---O--------------
-------------------------Q------------------------
--------------------------------F-----------------
-------------------------T------------------------

"59 Loose"
---------------S------M--------W-----------
----------E--------T------T--------E-----------
-----------O---O---O---C---O---O--------------
-------------------------Q------------------------
--------------------------------F-----------------
-------------------------T------------------------