LENGTH
The first number times two always denotes the nominal length:
Example:
4W = 4″ x 2 = 8″ long
5J = 5″ x 2 = 10″ long
6T = 6″ x 2 = 12″ long
8W = 8″ x 2 = 16″ long
HEIGHT
The first letter denotes the unit’s nominal height:
S = 2 5/8″
J = 3″
P = 4″
T = 5 1/3″
W = 8″
Thus:
4S = 2 5/8″ x 8″
5J = 3″ x 10″
6P = 4″ x 12″
6T = 5 1/3″ x 12″
4W = 8″ x 8″
8W = 8″ x 16″