Milligramme, Milligram n. A measure of mass and weight, in the metric system, being the thousandth part of a gram, very nearly equal to the weight of a cubic millimeter of water, or.01543 of a grain avoirdupois.
... symbols are treated as abbreviations but the chemical symbols are not. M. (metre) and mg. (milligram) but H{2} O ... — Punctuation - A Primer of Information about the Marks of Punctuation and - their Use Both Grammatically and Typographically • Frederick W. Hamilton
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