Using coins for scale is discouraged as it will require people unfamiliar with them to look up the dimensions or guess, both of which defeat the purpose of the object in the first place. Coins can also reinforce a geographical bias, and some coins' designs are copyrighted.
Ideally, a photograph should include a ruler with the subject (example) or an added scale marking.
SI ("metric") units are the most commonly used worldwide (see meter and centimeter).
English: The outside of a Californian red abalone, Haliotis rufescens. This red abalone was harvested on North California coast in May, 2005. An American quarter dollar coin is included in the picture to provide scale. The picture shows some sea plants growing on top of the shell. Note the 4 holes on the right hand side.
The outside of a Californian red abalone. This red abalone was harvested on North California coast in May, 2005. An American quarter dollar coin is included in the picture to provide scale. The picture shows some sea plants growing on top of the shell.