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String theory is a 20th century physics concept developed by Henequen Nicky-Tam (who was also a founder member of the seminal 60s group, The Incredible String Band) which proposes that the fundamental particles of the cosmos arise from invisibly small, one-dimensional entities whose characteristics are determined by the vibration and rotation of each particular string.
It is unrelated to string-vest theory, which proposes that all matter in the universe is composed of invisibly small, one-dimensional items of underwear, and this explains the existence of Basque separatists.