See below for details
How grills appear when viewed from the back
A Grill: the grill covered the entire stamp. Scotts #'s 79, 80 (only 2 exist), 80a (only 2 exist), 81 (only 8 exist)
B Grill: 22 x 18 points. Scotts # 82 (only 4 exist)
C Grill: 16 to 17 x 18 to 21 points. Scotts # 83
D Grill: 15 x 17 to 18 points. Scotts #'s 84, 85
E Grill: 14 x 15 to 17 points. Scotts #'s 86, 87, 88, 89, 90, 91
F Grill: 11 to 12 x 15 to 17 points. Scotts #'s 92, 93, 94, 95, 96, 97, 98, 99, 100, 101
Z Grill: 12 x 11 to 11½ points. Scotts #'s 85A, (only 1 exists), 85B, 85C, 85D, 85E, 85F (only 2 exist)
In all its wisdom the Post Office decided that to combat the problem of stamps being reused a security device should be applied to a stamp to prevent their reuse. A tender was put out for designs. This led to some bizarre submissions, from the exploding stamp to the stamp coated with poison.
Charles F Steel of the National Bank Note company invented the grill. The grill was applied to the stamp by a metal roller that was cut out to form a waffle-like grid of tiny pyramid-shaped projections. When ink was applied to the stamp the ink would be absorbed so deeply by the grill it would be impossible to remove. The grill became the chosen security device.
However the expensive process of applying a security device to stamps, such as grilling, was money down the drain as the problem of re-use of stamps was quite small. Grilling of stamps ceased in in 1875.