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).
“These teeth come from a dinosaur known as Troodon that lived during the Late Cretaceous Period (65 million years ago.). Paleontologists think that Troodon was a swift dinosaur, with sickle shaped claws, serrated teeth and a great deal of intelligence – making it a formidable carnivore for its size. (It was about 3 feet tall.) These shed teeth were found among the bones of several duckbill dinosaurs in the badlands of South Dakota.” The Children’s Museum of Indianapolis, IMCPL Digital Collections
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{{Artwork |artist= |title= |description= Troodon teeth (Troodon formosus) in the permanent collection of The Children’s Museum of Indianapolis |date=65 mya |medium= |dimensions= |institution={{Institution:Th