This is a virtuosic Romantic-era concert etude designed specifically to showcase mastery of octaves. It features dense textures and demanding athletic passages characteristic of the late nineteenth-century bravura style.
At Henle 7 this is a late-advanced work — the technical demands are serious and sustained. The main demands: octave passages that demand a relaxed wrist to avoid tension; fast passage work that stays light and even at tempo; and fine finger independence within the hand.
Beyond that, expect big chords that need weight without harshness.
Maintain consistent wrist flexibility to prevent fatigue while executing the rapid octave passages.
Comparable difficulty and character — good candidates for what to learn next.
A level or two down with the same spirit — ideal if this piece is still out of reach.
Where this piece's skills lead: slightly harder works making similar technical demands.
Etude de concert en octaves (Op.21) is rated Henle level 7 of 9 — late advanced repertoire, roughly ABRSM grade 8 and above. In the RCM system it corresponds to about level 10.
You should be comfortable at Henle level 7 — meaning you can already play level 6–7 pieces cleanly. Learning it one level early is possible as a stretch piece, but more than that usually leads to months of frustration.
A typical performance of Etude de concert en octaves (Op.21) lasts about 4 minutes.
Etude de concert en octaves (Op.21) is available as free public-domain sheet music on IMSLP — the score is linked directly from this page.
Pieces at a comparable difficulty and character include Eroica Variations Op.35 by Ludwig van Beethoven, Piano Sonata No. 21 'Waldstein' by Ludwig van Beethoven, Capriccio alla soldatesca by Charles-Valentin Alkan — see the full similar-pieces list on this page.