Most of Felix Mendelssohn's Opus 35, a set of six prelude-fugue pairs for piano solo, was written during the mid-1830s -- starting in about 1832 and gaining momentum as the 1837 date of publication approached. Op. 35, No. 1 is, however, an exception to this; its fugue seems to have been composed all the way back in 1827. It is often listed as the Prelude and Fugue, Op. 35, No. 1, in E minor/E major. But this is misleading, implying as it does that the prelude is in E minor and the fugue in E major. In fact, only at the tail end of the fugue does Mendelssohn make the switch from minor to major, and it is better to keep things simple and just say that it is in E minor.
Unlike the fugue that follows it, the prelude of Op. 35, No. 1 has never found a chronological home; no one seems quite sure when Mendelssohn wrote it, although 1832 or 1833 seem the most likely candidates. It is florid, and thick with always-rising arpeggios that fall nicely to the hand. Amidst this translucent Allegro con fuoco texture there is an angst-filled middle-voice melody; every so often a moving bass line pops up to support the froth above.
Mendelssohn was keen to trample on some of the previous few centuries' hallowed rules of melody-construction when he sat down to write the subject for the fugue: tritones abound, and the likewise forbidden intervals of the augmented second and the diminished fourth(!) find their way in as well. The fugue is in four voices, and reaches a climax in sixteenth notes that gives way at the end to a "Chorale" in E major.