How did Mahler and Carl Nielsen use progressive tonality in symphonic form?

Gustav Mahler and Carl Nielsen utilized progressive tonality as a significant modification to traditional sonata form. While the Classical sonata relied on the conflict and reconciliation of keys within a stable tonal framework, progressive tonality treats the home key as a goal to be reached from more distant tonal regions.1

Consequently, symphonies employing this technique conclude in a different key than the one in which they began. Mahler and Nielsen arrived at this concept independently during the late 19th century, with notable early examples including Nielsen’s Symphony No. 1 in G Minor (1891–92) and Mahler’s Symphony No. 2 (Resurrection, 1888–94). By shifting the structural focus of the symphony, both composers were able to forge highly individual forms that moved beyond the traditional reliance on the exposition-development-recapitulation principle of key contrast.12