The first problem in the hobby and the one that stops beginners. Great Britain issues no country name at all, by right of having been first; everyone else uses something, and the something is often not in English.

The single most important technical measurement in the hobby, and the one that most often separates a common stamp from a rare one. Two stamps identical in every visible respect can differ enormously in value on a perforation count.

Invisible in normal handling and frequently the difference between a common stamp and a scarce one. The same design printed on watermarked and unwatermarked paper is two catalogue entries.

Applied to change a stamp's value, its country of use, or its purpose. Overprints are among the most faked things in the hobby, because adding ink to a common stamp is cheap and the result can be worth a great deal.

Distinguishes issues that look alike, and separates genuine stamps from forgeries. It needs nothing more than a lens and a fingertip.

One of the most heavily studied series in the world, issued continuously since 1967 in hundreds of variations. To a casual eye they are all the same stamp; to a specialist there are thousands of distinct entries.

The bulk of American philately, and the area where perforation, watermark and printing variety do almost all the work. The same portrait can appear across dozens of catalogue numbers.

A hugely popular collecting area covering dozens of territories issued to a common template. Perforation and watermark varieties within a single colony's issue are extensive.

The category that accounts for a large share of 'I found a rare stamp' discoveries. Christmas and charity seals, advertising labels, exhibition labels and local carriage issues all look like stamps and are not.

Genuine government issues that were never valid for postage, and a substantial collecting field in their own right. They turn up constantly on old documents and get mistaken for postage stamps.

The category everyone hopes to find and the one most often misidentified. There is a real distinction between a catalogued error, a printing freak and ordinary damage, and it decides whether something is worth a great deal or nothing.

The largest single value factor in the hobby after identification, and the one beginners consistently underestimate. The same catalogue number in poor and superb condition can differ by a factor of a hundred.