OK, so is it the original content that distinguishes carboys from demijohns? Are my 2.5 and 5 gallon glass water "carboys," that close only with a #7½ (7-and-a-half) stopper, really demijohns? Does that mean that they hold half of a "john"?

I always wondered about that line from The Music Man that talked about "hogsheads, casks, and demijohns."

But I also have a pair of 6½ (6-and-a-half) gallon acid carboys, shown above, which close with a nylon screw cap and also accept a stopper. Did they switch from being a carboy to a demijohn when I filled them with fermenting cranberry juice? The mind reels…

Those guys get really heavy when full! But I recently found a gizmo called "The Brew Hauler" at the local wine and beer supply store that helps considerably.

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Terry Monteith wrote: The fact that you mentioned 'carboys', and their use in holding apple cider, made me reflect on proper terminologies, and how the terms can change after a hundred or more years of misuse. When carboys were invented a couple thousand years ago, and up through the 19th century, they were made to hold caustic materials, i.e....acids, solvents, etc...basically any liquids that were unpotable. 'Demijohns' were used for potable liquids such as your cider, wines, liquors, and such.