doublet

Doublet is a term used across disciplines to denote a pair of related items or a unit consisting of two closely connected parts. The word comes from the French doublet, meaning a pair. Its precise meaning depends on context, including fashion, science, linguistics, or puzzles.

In science, a doublet often refers to a quantum-state or a spectroscopic feature. A doublet state has total spin S = 1/2, corresponding to two possible spin projections. In spectroscopy, a doublet describes two closely spaced lines produced by interactions such as spin-spin coupling or fine structure. Doublets appear in electron spin resonance, atomic emission, and molecular spectra, and they reveal internal interactions within the system.

In historical fashion, the doublet was a close-fitting jacket worn by men from the 14th to the 17th century. It was often padded and worn with hose, forming part of an early form of the suit. The term reflects its design as a garment that was effectively a doubled or layered upper body garment.

In linguistics, a doublet is a pair of words in the same language that derive from different etymons yet remain related in meaning or form. The concept illustrates how languages borrow from multiple sources and develop parallel terms that converge in sense.

Other uses include the word puzzle Doublets, popularized by Lewis Carroll. In this game, a starting word is transformed into a target word by changing one letter at a time, with every intermediate step required to be a valid word. This puzzle emphasizes word connection and transformation.