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No, a ton is not a ton is not a ton. In fact, a grain is a grain is a grain is the correct answer. There were three measures of weight based on the grain (actually based on the weight of a seed of barley, I think), the avoirdupois, troy, and apothecary systems. The apothecary system was fairly recently dropped.
Although the grain is the same in each system, ounces, pounds, and tons diverge. And then there are short and long tons, and tonnes.
Bob
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