This is the first movement from Tchaikovsky's Piano Concerto No. 1, often performed in a piano transcription. It is known for its grand, virtuosic opening and sweeping melodic lines.
Requires significant stamina and the ability to articulate rapid passages clearly.
Allegro ma non Tanto in G major 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 9.
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 Allegro ma non Tanto in G major lasts about 10 minutes.
Allegro ma non Tanto in G major is available as free public-domain sheet music on IMSLP — the score is linked directly from this page.