The Symphony, Op. 21, was the first large-scale orchestral work Webern had written since the Five Pieces, Op. 10, 15 years earlier. The work marks the beginning of a period of extreme compression in Webern's music. Dedicated to his daughter Christine, the Symphony is a work of severe economy and restrained expression. Its symmetrical structure and pointillistic texture are quintessential hallmarks of Webern's mature compositional style.

Scored for clarinet, bass clarinet, two horns, harp, first and second violins, viola, and cello, the Symphony is widely regarded as a masterpiece in miniature: Webern's teacher and mentor Arnold Schoenberg was astounded and moved by the work's concision. Like most of Webern's 12-tone works, the Symphony is based on a single series dominated by semitones. The work consists of two short movements. The first is in two parts—statement and development—and begins with a double canon in four parts; the second movement is a theme with seven variations and a coda, and also includes the use of canon.

The Symphony is perhaps most remarkable for its use of symmetry, which in some quarters has stirred accusations against Webern of a certain excessive pedantry. That symmetry takes several forms, from the work's palindromic series to the canonic variations that work in both directions from the exact center of the piece outwards. The astute listener can spend a lifetime hearing an intricate web of such structural correlations within the Symphony, which is a sort of super palindrome.

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