In this connection 'different key' means a different tonic, rather than merely a change to a different mode (see: Picardy third and List of major/minor compositions): Gustav Mahler's Second Symphony (1888–94), for example, which moves from a C minor start to an E-flat major conclusion, exhibits 'progressive tonality'—whereas Ludwig van Beethoven's Fifth Symphony (1804–08), which begins in C minor and ends in C major, does not.
Progressive tonality in the late nineteenth century no doubt reflects the increasingly programmatic and narrative orientation of 'late Romantic' music.
Vocal music, with its explicit and verbally defined narrative and programmatic allegiances, perhaps featured the initial exploration of 'progressive tonality.'
Single operatic 'numbers' which (usually for some discernible dramatic and expressive purpose) fail to return to their original tonics can also be found—while in the quartets, symphonies and sonatas of the time such a practise was exceedingly uncommon.
As in his symphonies, Mahler took the idea of 'progressive tonality' in the song cycle to an extreme of refinement: each of his four Lieder eines fahrenden Gesellen ends in a key other than its original tonic.
The distinction was not one of chronological or stylistic 'advancedness', but rather a means of distinguishing between tonal motions that could either be reckoned as 'up' or 'down' around a circle of fifths construed as intersecting with the original tonic.
The same period showed a quickening of interest in 'progressive tonality' as displayed in the music of Carl Nielsen, in which it plays a particularly significant role.
In Nielsen's Fourth Symphony, for example, the initial tonal focus of D minor (clashing with C) issues at the end of the work in a firm E major.
This huge six-movement, two-part work begins with a sonata movement in D minor whose second-subject area is initially D-flat, becoming C-sharp; this moves to E in the matching portion of the recapitulation.