A picture showing one grain of pure gold and size comparisons with a 5 British pence, 20 euro cent, 10 yen and 25 U.S. cent coins. Also shown is a tape measure with Metric units on the bottom showing the diameter of the grain gold piece. The measured diameter is 3.18 mm or 1/8 inch.
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).
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