The length of any glove is historically measured in "buttons", which dates back to a time when buttons on gloves were intentionally sewn to be precisely 1-inch apart.
Button length refers to the distance measured from the base of the glove thumb to the topmost part of a glove.
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2-button: 8 - 9 inches in length, reaching just to the wrist without passing it.
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4-button: 10 - 11 inches in length, covering the wrist and extending several inches up the forearm
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6-button: 12 - 13 inches in length, extending along the forearm and often including a gauntlet-style sleeve
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8-button: 14 - 15 inches in length, reaching just to the elbow without passing it.
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12-button: 18 - 19 inches in length, reaching just past the elbow.
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16-button: 22 - 23 inches in length, typically ending towards the middle of the upper arm. this glove length represents the classic "opera length glove".
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21-button: 27 - 29 inches in length, reaching all the way to the shoulder.