US Stamp Perforations

The following are the most common U.S. Bureau of Engraving and Printing (BEP) line stamp perforations:

Flat plate perforation 10 (K-79)
Rotary press perforation 10 (K-80)

Rotary press perforation 10.5 (K-75)

Flat plate perforation 11 (K-72)
Rotary press perforation 11 (K-70)

Flat plate perforation 12 (K-66)

Below is a picture of a stamp reference set that I put together for the above perforation measurements.

For some of the other collectors that don't specialize in US / Canadian Stamps the (KXX) after the perf measurement is a reference to the Kiusalas perf gauge.
A visual of the Kiusalas perf gauge can be seen here.

https://goscf.com/t/9828

U.S. Stickney rotary press perforation 10 x 10 stamps were perforated with a Stickney bar and wheel perforator. The perforation 10 horizontal was perforated with perforating bars that actually measure around 9.84 perforations per 2 cm which corresponds to Kiusalas 80 (0.08" between perfs). However, the perforation 10 vertical was perforated with perforating wheels that actually measure around 9.87 perforations per 2 cm which does not correspond to any Kiusalas number. This is one example of how having a copy of the actual stamp itself can help to more accurately measure perforations.

Flat plate stamps and rotary press stamps have different perforation measurements because they were perforated by different perforating machines, especially for those measurements that appear to be the same but are actually not like perforation 10 and perforation 11, etc.