Why? Tagging is a tool for postal automation (such as automated facing and canceling mail).

How? When a stamp is exposed to short/long wave ultraviolet radiation, the phosphorescent/fluorescent ingredients used on stamp will glow. This glow can be detected by various devices used in mail handling.

How to detect? Place stamp under black light / UV-lamp, and presto. Some forms of tagging can be seen also with bare eye, when looking stamps in proper angle against a light source…. I definitely recommend buying a black light, as it’s highly useful for detecting repairs, mold etc. too.Various countries have used different types of tagging ingredients, which is worldwide collectors should buy so called “dual mode” black light that emits both short and long wave UV. Lighthouse makes some battery powered models that I can recommend highly… I recall Canadian stamps require short wave only, but I’m sure someone will correct if I’m wrong.

And finally… When using a black light, remember to follow safety precautions. Though these are low power devices (when battery operated), they can still cause some serious damages if not handled properly.