The verdict of Von Bulow on the following study in A flat, No. 10, has no uncertainty of tone in its proclamation: 

He who can play this study in a really finished manner may congratulate himself on having climbed to the highest point of the pianist’s Parnassus, as it is perhaps the most difficult piece of the entire set.  The whole repertory of piano music does not contain a study of perpetuum mobile so full of genius and fancy as this particular one is universally acknowledged to be, except perhaps Liszt’s Feux Follets.  The most important point would appear to lie not so much in the interchange of the groups of legato and staccato as in the exercise of rhythmic contrasts—­the alternation of two and three part metre (that is, of four and six) in the same bar.  To overcome this fundamental difficulty in the art of musical reproduction is the most important thing here, and with true zeal it may even be accomplished easily.

Kullak writes:  “Harmonic anticipations; a rich rhythmic life originating in the changing articulation of the twelve-eights in groups of three and two each. ...  This etude is an exceedingly piquant composition, possessing for the hearer a wondrous, fantastic charm, if played with the proper insight.”  The metronomic marking is practically the same in all editions, 152 to the quarter notes.  The study is one of the most charming of the composer.  There is more depth in it than in the G flat and F major studies, and its effectiveness in the virtuoso sense is unquestionable.  A savor of the salon hovers over its perfumed measures, but there is grace, spontaneity and happiness.  Chopin must have been as happy as his sensitive nature would allow when he conceived this vivacious caprice.

In all the editions, Riemann’s excepted, there is no doubt left as to the alternations of metres.  Here are the first few bars of Von Billow’s, which is normal phrasing: 

[Musical score excerpt]

Read Riemann’s version of these bars: 

[Musical score excerpt]

Riemann is conducive to clear-sighted phrasing, and will set the student thinking, but the general effect of accentuation is certainly different.  All the editors quoted agree with Von Bulow, Klindworth and Kullak.  But if this is a marked specimen of Riemann, examine his reading of the phrase wherein Chopin’s triple rhythm is supplanted by duple.  Thus Von Bulow—­and who will dare cavil?

[Musical score excerpt]

Riemann: 

[Musical score excerpt]

The difference is more imaginary than real, for the stems of the accented notes give us the binary metre.  But the illustration serves to show how Dr. Riemann is disposed to refine upon the gold of Chopin.