Elliott Carter’s string quartets feature some of the composer’s most innovative, personalized, and boldest ideas. The first three quartets (1951, 1959, and 1971) were particularly exploratory in nature, leading to the development of techniques that mark Carter’s mature and late periods—harmonic language based on all-interval tetrachords, dense textures containing multiple polyrhythmic strands, complex counterpoint, individualization of characters, spatialization, and novel formal designs.  Carter attributes the inception of his rhythmic expression to the techniques of Ives, Stravinsky, and Nancarrow, composers which he quotes, some more explicitly than others, in his First String Quartet.  However, a close study of the sketch material, housed at the Paul Sacher Stiftung in Basel and the Library of Congress, reveals that the works of other composers, namely Bartók and Webern, served as an inspiration and even the conceptual point of Carter’s Second Quartet.  While Carter did not specifically discuss these composers’ impact on the development of his own expression, the sketches show careful reworking, re-composing and disguising of segments from Bartók’s Third String Quartet and Webern’s Bagatelle No. 6.  In this essay, I examine the purpose, function, meaning, and different uses of existing music in Carter’s early quartets, following the typology set forth by J. Peter Burkholder in his studies on musical borrowings.