The words orthodontics and orthodontist were both modelled after orthodontia, which was created in 1849 from the Greek word orthos, meaning "straight", odon, meaning "tooth", and the noun-forming suffix -ia. Orthos, the root in words like orthopedic and orthography, comes (through Proto-Hellenic ortwos) from the Proto-Indo-European root herd, meaning "upright". Odon (the origin of the second part of the word mastodon) traces, through Proto-Hellenic odonts, to Proto-Indo-European hdonts, which still meant "tooth". Herd is also the etymon of English arduous, and hdonts of English tooth. After being popularized around the turn of the twentieth century, usage of the word orthodontia peaked in the early 1920s and late 1930s, and has been decreasing since due to being overtaken by orthodontics.
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