Murray Gell-Mann won the 1969 Nobel Prize in Physics for contributions and discoveries concerning the classification of elementary particles and their interactions.
Gell-Mann developed the Eightfold Way, a scheme that organized many strongly interacting particles into mathematical patterns. In 1964, he proposed that hadrons such as protons and neutrons are built from more fundamental constituents called quarks. George Zweig independently proposed a similar model.
The quark model initially used three flavors: up, down and strange. Later experiments confirmed additional flavors, eventually producing the six-flavor framework used in the Standard Model. Quarks are not normally observed in isolation because of confinement.
The 1969 prize recognized Gell-Mann’s broad work on particle classification and interactions, not the discovery of every quark. A common confusion is attributing the quark model solely to Gell-Mann; Zweig’s independent contribution is historically important, although Zweig did not receive the Nobel Prize.