Probably the most referenced measurement in stamps are the watermark and the perforation size. Perforation gauges as seen above cost just a few dollars and are an invaluable tool in philately. By placing the stamp, as shown above against the various illustrations of perforation gauges you can see which gauge matches the perforation on your stamp. In the example above the stamp is a perf 10 on the top and on the side. On the Washington-Franklin series it is a good idea to measure both the vertical and horizontal sides as some of the series are perforated 11 x 10, 10 x 11, etc. The top perforation is always quoted first in stamp catalogues.