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FLAT PLATE PRINTING in the United States and territories under the jurisdiction or administration of the United States.
Flat plate press was initially used for printing all stamps of the United States. The first engraved plates used for this purpose in 1847 contained 200 subjects. The number of subjects to a plate varied between 100 and 300 until the issue of 1890, when the 400-subject plate was first laid down. Since that time, this size of plate has been used for a majority of the regular postal issues (those other than commemoratives).
Most stamps were printed and sold by postmasters in whole sheets, which means that sheets, per se, are not uncommon. Because the resulting sizes were often so large as to make them unwieldy, these large sheets were typically cut into counter sheets, also called panes.
The sheets of many classic U.S. stamps had only two panes, many of the Bureau issues had four panes, while modern sheets may have six or more panes. A white margin surrounds the stamps on the four sides.
Voids called gutters separated the panes, giving printers waste-free areas within which to make their cuts. When it comes to the traditional, perforated sheets, collectors look for the colored sheet numbers in the margin or gutter, which can identify a sheet by its plate block, as well as its print run.
In a typical 400-subject plate there were guide-lines: horizontal or vertical colorde lines, extending wholly or partially across the sheet. They serve as guides for the operators of perforating machines or to indicate the point of separation of the sheet into panes.
When guide lines are used to mark the division of the sheet into panes, the space between the stamps at the edge of the pane is no different than the space between any other stamps on the sheet. Some plates provide a wide space, or gutter, between the panes. These plates do not produce guide lines.
As a result of cutting sheets in two or four panes, these panes always contain two imperforate sides:
Pane 1: one on the right side and one at the bottom;
Pane 2: one on the right side and one at the top;
Pane 3: one on the left side and one at the bottom;
Pane 4: one on the left side and one on the top
Stamps in the corners always have two sides imperforate.
The rotary press printing was developed in 1910 and first production took place in 1914. Because of the curved plates of the rotary press cylinders, the rotary impressions are somewhat larger.
Sources of information:
Scott Specialized Catalogue of United States Stamps & Covers
Stanley Gibbons Stamp Catalogue part 22 United States
N.B. The use of illustrations isn’t yet available on this background page. Those illustrations would have been very useful and would have made the text more understandable.