Cavaillé-Coll instrument

The type of large French Romantic organ built by Aristide Cavaillé-Coll (1811–1899), for which virtually all of Franck's organ music was conceived. Characterised by powerful Reeds, expressive swell division, and orchestral voicing.

Franck's organ music must be understood in relation to the Cavaillé-Coll sound — rich, sustained, and orchestral. On a smaller instrument, registration must be adjusted to approximate these qualities. The key characteristic to preserve is the ability to make long, gradual crescendos and diminuendos, which the swell box enables. Without this, the architecture of works like the Choral No. 2 collapses.