Carriage bolt pertains to a bolt with a round head and square shoulder. The square shoulder gets pulled into the hole and keeps the bolt from turning when the nut is being tightened.
The round head doesn't need a washer, has no edges to scrape or catch things and gives a finished look.
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Carriage (Carriage is described as the fixture in the ...)
Alternative Spellings (Synonyms): carriages, bolters, Bolts