Plate Markings
The denomination, in this case the "10c", was printed in both colors, often overlapping, reversed in the upper left margin and upside down and reversed in the lower right  margin.  This provided some protection against the inadvertent  printing of the wrong frame with the wrong vignette. On the lower denominations, further protection was added in the form of a small numeral near the denomination marker.

The seemingly random characters are the initials of the pressmen who worked with the plates. The initials may be either black or the color of the frame.

Below the initials are the imprint and plate number, always in the color of the frame. 

There are four imprints per sheet, two at top and two at bottom, meaning there are no left or right margin plate blocks. The plate number of the vignette, or black, plate occurs only once per sheet, below stamp 95 of the bottom pane just to the left of the arrow in the bottom margin (as in the diagram at right). Thus a plate block with both plate numbers is found only once per sheet - the bottom left plate block.

The 1901 Issue , often referred to as the Pan American Series, was printed on soft porous double-line USPS watermarked paper in the 200 subject layout shown below. The sheet was cut along the "cut line" into two panes of 100 stamps  before distribution to post offices. This issue presents many fascinating aspects in plate design and layout and is an excellent one to start our study of the plate layouts on U.S. stamps.

Guide Lines and Arrows
Although there are guide lines in both the vertical and horizontal direction, only the horizontal guide line was actually cut.

This had the interesting effect of creating straight-edge stamps along the cut-line, meaning straight-edges were possible only in the top or bottom margins of the stamp.  Note the guide lines on the two stamps at right. The top stamp was from the bottom row of the top pane and the bottom stamp from the top row of the bottom pane.

The fact that there are no straight-edges at the sides is important to the collector, since re-perforation at left or right is therefore rendered unlikely. Conversely, the collector should be aware that re-perforation at top or bottom is possible, especially on the higher value stamps.