Bach's "371 Harmonized Chorals" is a foundational collection that showcases his unparalleled skill in harmonizing German chorale melodies. These pieces are essential for developing a deep understanding of Baroque counterpoint and harmonic language.
Developing a keen ear for voice leading and smooth harmonic transitions is crucial for performing these chorales effectively.
371 Harmonized Chorals is rated Henle level 6 of 9 — advanced repertoire, roughly ABRSM grades 7–8. In the RCM system it corresponds to about level 9.
You should be comfortable at Henle level 6 — meaning you can already play level 5–6 pieces cleanly. Learning it one level early is possible as a stretch piece, but more than that usually leads to months of frustration.
371 Harmonized Chorals is available as free public-domain sheet music on IMSLP — the score is linked directly from this page.