Many stamps were printed several times over the years with the same design, and the only visible differences are the perforations or a watermark hidden in the paper. Catalogues list these as separate stamps, and their values can be very different. Here is how to check both.

Perforation gauge: what "perf 14" means

The perforation measurement is the number of holes (or teeth) in 2 centimetres. A stamp described as "perf 14" has 14 holes per 2 cm. "Perf 11½" has 11½.

To measure:

  1. Place the stamp on a perforation gauge, a card or clear plastic ruler printed with dot rows for each size. Gauges are cheap from any stamp dealer.
  2. Slide the stamp along until the teeth on one edge line up exactly with one row of dots.
  3. Read the number on that row.
  4. Measure the top and a side, because they can differ. A stamp can be "perf 14 × 13½", with the horizontal value given first.

Other separation methods

  • Imperforate. No perforations, cut apart with scissors. Early stamps before about 1855 were all imperforate. For later stamps, check the margins: an "imperf" can simply be a perforated stamp that has been trimmed.
  • Rouletted. The paper is slit or pricked rather than punched out, so no holes are missing.
  • Die-cut. Self-adhesive stamps with wavy or straight cut edges. They are measured the same way as perforations.
  • Straight edge. One or two unperforated sides, typical of booklet, coil and sheet-margin stamps.

Finding a watermark

A watermark is a design, such as a crown, letters, a star or a pattern, pressed into the paper when it was made. It shows up as thinner areas in the paper. Ways to see it:

  • Hold it up to a light, or lay it face down on a dark surface. This works for strong watermarks.
  • Watermark tray and fluid. Place the stamp face down in a black tray and add a few drops of watermark fluid. The watermark appears as dark lines for a few seconds. The fluid evaporates without harming most stamps, but avoid it on photogravure stamps and some modern inks, which can run, and on self-adhesives.
  • Electronic watermark detectors use light filters and are safe for any stamp.

Note whether the watermark is upright, inverted, sideways or reversed, because catalogues list these separately and some are scarce.

Worked example

British Machin definitives, and many Commonwealth stamps from the King George V and VI era, exist in many printings that differ only in perforation, paper and watermark. Two stamps of identical colour and design can be worth very different amounts depending on these details, which is why specialists always check them.

Put it together

Identify the country and issue first (here's how), then use perforation and watermark to pick the exact catalogue number. AI Stamp can identify the issue from a photo, and your notes on perforation and watermark stay with the stamp in your collection.