The purpose of grilling stamps was to break the fiber of the stamp paper so that when a cancel was applied the ink would soak into the paper and make washing the cancel off much more difficult. A grill is essentially an embossment of the stamp paper, in the form of a very small "waffle" pattern. (See left, illustration below.)

    The first grills used were an allover design, which embossed the waffle pattern over the entire stamp. (This grill type is an A grill, used in 1867.) However, since grilling broke the paper of the stamp, allover grilling made the stamps very fragile to handle, and almost impossible to separate cleanly at the perforations. A decision was made to reduce the size of the grill to cover only the area in the center of the stamp. The B grill and C grills were introduced also in 1867, followed by the D, Z, E, F, G, H & I grills, all of which were of slightly different sizes.
    Note: The first three grill types (A, B & C) are known as points-up because the pyramid-shaped embossments created by the grilling process point up from the surface of the stamp. The rest of the grill types (D, Z, E, F, G, H & I) are points down, i.e. the pyramids point downward into the surface of the stamp. This is largely because points-up and points-down grills were created using two different processes. While each points-up grill embossment may show, when magnified, a perfect pyramid, the center of each points-down grill embossment shows, when magnified, either a vertical or horizontal ridge, as illustrated above, at right. Only the Z grill shows horizontal ridges; all other points-down grills (D, E, F, G, H & I) show vertical ridges.
    Below is a table that shows which grills were applied to which stamps. Links within the table will provide an image of each stamp design listed.

Grill Type   Point Count (horizontal  
then vertical)
Issue Type: Scott stamp
  design number in [brackets]  
Scarcity   Scott Catalog  
A, POINTS UP All over 3 cent Washington [A25] Rare 79
" " 5 cent Jefferson [A26] Only 8 copies known 80, 80a
" " 30 cent Franklin [A30] Only 8 copies known 81
B, POINTS UP 22 across BY 18 down 3 cent Washington Only 4 copies known 82
C, POINTS UP   16 or 17 across BY 18, 19, 20 or 21 down   3 cent Washington Rare 83
D, POINTS DOWN 15 across BY 17 or 18 down 2 cent Jackson [A32] Rare 84
" " 3 cent Washington Rare 85
Z, POINTS DOWN 13 or 14 across BY 18 down 1 cent Franklin [A24] Only 2 copies known 85A
" " 2 cent Jackson Rare 85B
" " 3 cent Washington Rare 85C
" " 10 cent Washington [A27] Only 6 copies known 85D
" " 12 cent Washington [A28] Rare 85E
" " 15 cent Lincoln [A33] Only 2 copies known 85F
E, POINTS DOWN 14 across BY 15, 16 or 17 down 1 cent Franklin Uncommon 86
" " 2 cent Jackson Uncommon 87
" " 3 cent Washington Common 88
" " 10 cent Washington Uncommon 89
" " 12 cent Washington Uncommon 90
" " 15 cent Lincoln Uncommon 91
F, POINTS DOWN 11 or 12 across BY 15, 16 or 17 down 1 cent Franklin Uncommon 92
" " 2 cent Jackson Common 93
" " 3 cent Washington Common 94
" " 5 cent Jefferson Uncommon 95
" " 10 cent Washington Fairly common 96
" " 12 cent Washington Fairly common 97
" " 15 cent Lincoln Fairly common 98
" " 24 cent Washington [A29] Uncommon 99
" " 30 cent Franklin Uncommon 100
" " 90 cent Washington [A31] Scarce 101
G, POINTS DOWN 12 across BY 11 or 11 down "Pictorial" Issue of 1869 Common to scarce 112-122
  H & I, POINTS DOWN   10 to 13 across BY 10 to 16 down First "Banknote" Issue, 1870-71   Fairly common to rare   134-144

    Grills on the earlier issues were very well defined. However, grilling stamps after they were printed introduced an extra step in the production process that added costs and delays, and, in order to reduce both, the practice of grilling up to six sheets at a time was adopted during the printing of the large "banknotes" of 1870-71. As a result, the grill impressions on these stamps are usually faint and incomplete. Such poor grills could scarcely be said to be doing their job, and soon afterward the experiment of adding grills to US stamps was ended.