Why Does Brahms Make Triple Metre Sound Like Duple Metre?

Brahms often lets six equal note-values sound first as two groups of three, then briefly regroups them as three groups of two. This shift of accent is called a hemiola. The time signature does not change, and the underlying pulse does not vanish; what changes is our judgement of where each group begins. A settled triple metre seems to stretch sideways, so the music moves forward while resisting its previous grid.

In a passage from Schicksalslied, Op. 54, Brahms first establishes triple metre clearly, then creates a duple-like feeling through two-note groupings that cross the bar lines, before triple metre becomes explicit again. The point is not merely to identify a device. We should hear two organisations of time overlapping: the old metre continues underneath while a new accent pattern passes across it. Much of Brahms's rhythmic weight comes from resistance that bends an order without destroying it.

At the piano, the common mistake is to turn the new grouping into a row of heavy accents. It is better to preserve the original pulse and let the harmony, bass direction and phrase shape reveal the groups of two. The fingers may redirect the accents, but the body should retain the continuity of three. The result is not a rhythmic trick, but tension between two scales of musical time.

https://odp.library.tamu.edu/stepstomusictheory/chapter/more-rhythm-meter/

https://mds.marshall.edu/music_perf/742/

https://imslp.org/wiki/Schicksalslied%2C_Op.54_(Brahms%2C_Johannes)