It all started with the Regulations of 1851 (regs. 101-112).
Rank was denoted by size of the epaulettes, the diameter of the gold bullion, and embroidered insignia. The epaulettes of regimental officers would have the number of the regiment embroidered in gold within a circlet of embroidered silver, one and 3/4 inches in diameter, upon clothe of the appropriate branch color.
Thus, for example, a captain would wear epaulettes with bullion 1/4 of an inch in diameter and 2 1/2 inches long, with two silver embroidered bars. A first lieutenant would wear epaulettes with 1/8 inch bullion and 2 1/2 inches long, with one silver bar.
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