A doublet is a man's snug-fitting buttoned jacket that was worn in medieval and Tudor times. Originally it was a mere stitched and quilted lining ("doubling"), worn under a hauberk or cuirass to prevent bruising and chafing. Then, like many other originally practical items in the history of men's wear, from the late 15th century onward it became elaborated enough to be seen on its own. In the early 1580s, Sir Philip Sidney, when governor of Flushing in the Low Countries, chose to be portrayed in his doublet, but still in a gorget, as if he were caught in the act of setting aside his armour to institute a civil government. (See portrait at Sir Philip Sidney.)

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A doublet in gemstones is a fake gem composed in two sections, such as a garnet overlaying green glass, with the join hidden by the mount, giving the effect of a fine emerald. Similarly a layer of opal may be glued to a jet foil, giving the impression of a rare black opal.

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A doublet is also one of two or more words of the same language that come from the same root. Doublets may be nearly synonymous, for example English pyre and fire; may have fairly different meanings, for example aperture and overture (the commonality behind the meanings is "opening"); or may even develop meanings that are in a sense opposites, for example host and guest. Doublets also vary with respect to how far their forms have diverged. For example, the resemblance between levy and levee is obvious, whereas the connection between sovereign and soprano is harder to guess from the forms of the words alone.

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In quantum mechanics, a doublet is a quantum state of a system with a spin of 1/2, such that there are two allowed values of the spin component, -1/2 and +1/2. See also triplet.