A stamp design includes several elements required for it to accomplish its purpose satisfactorily. Most important is the denomination indicating its monetary value, while international agreements require a country name on almost all types of stamps. The second required element, at least for stamps intended to be used on international mail, is the name of the country.
The use of portrait busts (of the ruler or other significant person) or emblems was typical of the first stamps, by extension from currency, which was the closest model available to the early stamp designers.
By far the greatest variety of stamp design seen today is in pictorial issues. The choice of pictorial designs is governed by a combination of anniversaries, required annual issues, postal rate changes, exhaustion of existing stamp stocks, and popular demand.
Many countries have specific rules governing the choice of designs or design elements. Stamps of the UK must depict the sovereign (typically as a silhouette), while US stamps may not visibly depict any person who has been dead for less than 10 years, except for ex-Presidents, who may appear on a stamp one year after their demise.
Sometimes designers include tiny elements into a design, sometimes at the request of the stamp-issuing authority, sometimes on their own. The US stamp honoring Rabbi Bernard Revel has a minute Star of David visible in his beard.
Stamp design has undergone a gradual process of evolution, traceable both to advances in printing technology and general changes in taste. Design “fads” may also be observed, where a number of countries tend to imitate each other. This may be driven by printing houses, many of which design and print stamps for multiple countries. For instance, although multi-color printing was always possible, and may be seen on the earliest stamps of Switzerland, the process was slow and expensive, and most stamps were in one or two colors until the 1960s.