U is for Underprints
These are, as you would expect, quite literally the flip side of overprints – additional printing underneath the main printed impression of a stamp, in some cases on the back rather than the front. They have been used for a variety of purposes.
The most common is as a security measure. Some issues had a pattern printed onto the paper before the stamp itself was printed, with the aim of making them harder to forge (the philatelic term for this is a burelage). The general idea here is that the pattern should be clearly visible but not obtrusive – an example would be the early issues of Denmark, where a pattern of wavy lines was used.
Underprints on the back have more varied uses. One of these is simply as an aid to accounting, for example Swiss coil stamps from the 1934-48 Landscape definitives which had a ‘control number’ printed on the back every five stamps. The stamps of some GB booklets sold at a discount to face value had ‘star’ underprints that identified them (in order to discourage stamps from the booklets being sold separately).
Another widespread use is for commercial advertising. The New Zealand ‘Adsons’ of 1893 are probably the most famous example of this, with advertisements for such things as coffee, chocolate, and Beecham’s Pills on the backs of the stamps. Post Offices have also used an underprint for additional text to complement the main subject of a commemorative stamp.
Finally, there are ‘private enterprise’ underprints, again for security reasons (especially before ‘perfins’ were used), some firms printed or handstamped their names on the backs of stamps. These underprints are often rare, as being added over the gum they tended to disappear when the stamp was soaked off, although for a while the British Post Office offered a service whereby underprinting could be done by the stamp printers before gumming.