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The crossover: 1925. In the Ngram data, "interlibrary loan" (closed) first overtakes "inter-library loan" (hyphenated) in frequency around 1925, and from that point forward it never looks back ...
...Other familiar words followed the same arc — lifestyle, boilerplate, doorbell, screwdriver, tailwind, rowboat, and postcard all had eras when they were encountered more commonly as open or hyphenated compounds.
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