Question About Perforations

I've been collecting for several years, but I never gave much thought to perforation variations of the older stamps. Now that I have a little more disposable income I have been purchasing many of the stamps that I could not afford when I began collecting, and I have found that information about perforations is hard to find.

Question: The perforation gauge I bought has 20mm written beneath the pictures of perforations. Does that mean when, for example, they say a stamp has a perf. of 12, that there are 12 holes per 20mm length?

Second, when the say a stamp is perf. 11½ X 12, is that 11½ along the top & bottom, and 12 on the sides, or something else?

I appreciate any help.

You have it correct already in your posting.

Perfs are always measured horizontally first, so 11x10.5 would be 11 horizontal and 10.5 vertical.
Perfs are measured by the number within 20mm, but the number of pins involved can make a real difference in how accurate they appear on a small gauge. The larger specialist gauge shows this, perfs are listed at 12-66 or 12-67 for example. Wider is better when it comes to gauges, it's always best to measure as many perfs as possible at one time.

Very good questions. I was unclear on some of this too.

Thanks

Theoretically, the perforation gauge can vary on all four sides, although I don't recall any examples of this. When the perfs vary on all four sides, the measurements are given for top, right, bottom & left in that order. The only US examples with strange compound perfs that come to mind are the 1922-25 issues where some perf 11 stamps have one edge (top or bottom) perf 10. These are, of course, rarities. For modern issues, the 1997 botanical prints come to mind, specifically 3128a. The stamp is die-cut 11.2 on top, bottom & right but die-cut 10.8 on left. The die-cut 11.2 right side is not continuous as there is an odd die-cut in the middle, where the two stamps above it in the booklet converged.