The fugue in this key from the Well-Tempered Clavier, Book 1 is the only one in 6/4 time.
The fugue in this key from the Well-Tempered Clavier, Book 1 is the only one in 6/4 time. An Allegro Vivace movement that opens with alternations between 9/8 and 12/8 is the finale of a Rachmaninoff work in this key, his Opus 1. An Adagio Sostenuto movement in this key is, in standard performance, the longest individual movement by duration in a Beethoven piano sonata, and precedes a movement containing a three-voice Allegro Risoluto fugue in a late-period piano sonata in B-flat. Those pieces are the slow movement of the "Hammerklavier" Sonata and Rachmaninoff's First Piano Concerto. Mozart's only use of this key was in the second movement of his Piano Concerto No. 23, K. 488. The ascending melodic scale of this minor key adds a D-sharp and an E-sharp. This key is the mediant of D major. A Haydn symphony in this key sees the players gradually exit in its last movement. FTP, name this key of the Farewell Symphony, the relative minor of A major, the minor key with three sharps. 
The fugue in this key from the Well-Tempered Clavier, Book 1 is the only one in 6/4 time.
F-sharp minor [do not prompt on or accept "F-sharp" or "F-sharp major" or anything that says "G-flat"] 