When did these terms take on a quantifiable meaning other than just generic kitchenware? When did cup start having the meaning “Use 3 1/2 cups of chicken stock” as opposed to “I drink water out of a cup”?
Also, why does a “cup”, (or for that matter, a “teaspoon” or a “tablespoon”) refer to the amount of volume that it does? For example, I’ve heard stories of a foot being decided upon because of how long a king’s foot was… Who decided on how much a cup was? And what was their reason?
All three terms, cup, teaspoon, and tablespoon, took on specific measurements in the 1700s. I believe the origin, in terms of specificity, was in medicine where more precision was needed.
I think the amount came from the typical size of the appropriate utensil in those days. I doubt there was some royal cup that set the standard.
There also seem to be differences between countries in the size of some of these measurements. A teaspoon seems to always be the same - 5mL, or 5 cubic centimetres - but a tablespoon in my US cookbooks usually refers to three teaspoons (15mL), while Australian and some Canadian books show a tablespoon as four teaspoons, or 20mL.
Similarly, a my US cup measure shows 240mL, while some other countries use 250mL. This could well be a result of America’s general use of the imperial rather than metric system of weights and measures.
Believe me, when you’re cooking 5mL mightn’t make much difference if you’re using cups, but if you’re measuring out tablespoons of herbs and spices and can have quite a dramatic effect on the end result.
Bon appetit 
American foods are packaged in ounces, other nations use metric measures.
If your recipe calls for one cup, in the US the eight ounce bottle has 236 ml. In a metric package it is probably 250ml. Calling a cup 240 ml is just a convienient round off that allows the use of one package. Same for 250 ml cups.
Even pharmacists consider an ounce to be precisely 30 ml. They know it is not exactly equivalent. But in a world where people keep grains, gills and firkins alive, you gotta compromise somewhere.
“How much time he saves who does not look to see what his neighbor says or does or thinks.” ~ Marcus Aurelius Antoninus ~
There’s also some other differences. In the American system, a pint is 16 fluid ounces, or two American cups. In the Imperial system which is sometimes used in England, a pint is 20 fluid ounces. The ounces are the same, but I’m not sure if an Imperial cup is eight or ten of them.
Sorry to disagree with Jeff, but Fannie Farmer is generally credited with standardizing measurements such as cups, tablespoons, teaspoons, etc., in the 1890s.
I’ve never seen cups used as a measurement in British recipies - they’ll just give the amount in ml/fluid ounces.
Wherein I dare disagree with DDG :eek: (but not really)
I have no doubt that Fannie Farmer standardized (and popularize) the use of precise measurements in cooking, but I’m not sure that she was the first one to use use the terms in question as specific measurements. I’m sure that she also helped define what was meant by a “teaspoon” or “cup.” I’d be curious to see the 1896 version of her cookbook to see how she went about defining the measurements.
This was not just an overnight thing, you know. She spent years working on this. The lady was a powerhouse, a typical late-Victorian reformer. She didn’t actually write the “Fannie Farmer” cookbook–none of those recipes are “hers”. It’s actually the “Boston Cooking School Cookbook”, the Boston Cooking School being the cooking school that she basically took over and made her own, through sheer force of will, using it as a springboard for her ideas about reforming American Domestic Science as expressed in the kitchen.
She got together with manufacturers and said, “Look, this is the size a ‘cup’ should be, and this is the size a ‘teaspoon’ should be.” Manufacturers, for their part, were delighted to comply, as it meant that they could make new products that not only immediately rendered their old products obsolete, but also made them look marvelously “scientific”, “with it”. Those graduated tin measuring cups date from this period, as do the sets of tin measuring spoons, 1/4 tsp., 1/2 tsp., etc.
She also pioneered the concept of “level” measurements, meaning you sweep off the top of the sugar or flour or salt or whatever, so it’s flat instead of heaping. This makes for much more accuracy.
Up till her time, recipes called for “a cupful” or “a spoonful”, or “a wineglass full”, or “a lump of butter the size of a walnut”. A “cup” was generally taken to mean a “teacup”, but different teacups hold different amounts. And how full were you supposed to fill it? Brimming over, or just to the lip? And for dry ingredients, most women usually scooped the flour out so that it was heaping, but too much flour in the cake batter makes for terrible doughy cake.
So, if you were a housewife following a Fannie Farmer recipe, and it called for 1/4 cup of sugar, you knew exactly how much sugar to put in, using the new tin measuring cups. The book was an instant best-seller, and a version of it has been in print ever since. And, if you really wanna know how she put it in the 1896 edition, you can go down to the bookstore or the library and look it up–it’s widely available in reprints. IIRC, she just says briskly, “These are the measuring cups and spoons you must use”, and there’s a picture of the tin cups and spoons. But I could be wrong–it’s been a while since I looked at it. I’ve got the 1923, 1959, and 1982 editions here on my shelf, but not 1896. Hmm, I sense a birthday present idea coming up, and it’s next week, too… 
Historical footnote: Not only did she standardize measurements, she also standardized the way American women cooked. Since everyone was using Fannie Farmer cookbooks, everyone was making things the same way, like potato pancakes. So when immigrants came over on the boat, with their own ethnic recipes for something like potato pancakes, they immediately scrapped them in favor of Fannie Farmer’s recipe, because they wanted to blend in and “cook like Americans”. Culinary historians have been cursing her ever since.