Towards the end of his life, Frédéric Chopin wrote a series of preludes; while there were 26 in all, the piece known as #25 (1841), although published earlier, was actually the final installment of the series to be written.
According the AllMusic, “This last prelude begins with a gentle, melancholy theme, whose mostly ascending accompaniment Chopin deftly works into the gloomy melodic fabric. That is nothing new for him, but here the obsessive and seemingly simple manner of the harmony never becomes tiring, always remains profoundly atmospheric, largely because of the composer’s manipulation of his thematic material, at times allowing sunlight to break in, as when the theme is played for the second time and blossoms into hopeful joy…It is hardly surprising that this masterpiece is one of Chopin’s more popular and widely-played compositions.”
Although we can see from this sheet music-centric video that the key signature never formally shifts, modulations and passing keys-of-the-moment are more the rule than the exception in this piece, calling legions of accidentals into service. AllMusic goes as far as to suggest that “the success of this whole piece rests on the composer’s deft handling of what is essentially threadbare thematic material — there is no middle section here.”