The United States Post Office changed the perforation gauge on flat plate stamp sheets from Perforation 10 (P10) to Perforation 11 (P11) in early 1917 because P10 was too difficult to separate without damaging the stamp.

This transition represented the final step in a decade-long trial-and-error process by the Bureau of Engraving and Printing to find a balanced perforation standard:

Perforation 12 (Used until 1914): The Post Office originally used P12, but the holes were punched too closely together. Heavy-use businesses complained that the sheets were too flimsy and brittle, frequently falling apart and tearing prematurely at the slightest touch.
Perforation 10 (1914–1917): In an attempt to strengthen the sheets, the Bureau switched to a wider P10 gauge. However, this went too far in the opposite direction. The holes were spaced too far apart, making the paper too tough to tear. Postal clerks and citizens frequently ripped or ruined the stamps when trying to separate them from the sheet.

Perforation 11 (Introduced 1917): P11 was implemented as the ideal "Goldilocks" middle ground. It provided enough structural integrity so sheets wouldn't fall apart during shipping, while remaining clean and easy enough to tear apart by hand.