The pound cake got its name because the original recipe called for exactly one pound of each of its four ingredients.
Why it is true
Originating in the eighteenth century, the name served as an easy mnemonic for people who couldn't read, using flour, butter, sugar, and eggs.
Source: Culinary Origins
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This is the reading side of the site, so the answer is in the sentence above — that is the whole point of it. Test yourself works the other way round: the question comes on its own, and the fact and the explanation only appear once you have answered.