10 Variations
In his piano sonata 30 and 32, variations are used as the finale. Observe that variations and fugues are actually forms used in the past and they are not the most popular form in Beethovens days. Yet, variation is a effective form to present a musical idea.
The coherence and, at the same time, the differences between variations acts like magic in Beethovens hand. Contrapuntal elements were quite heavily implemented into some of the variations.
A point to note is that in most of the late Beethoven piano work, extensive use of trills were used which often become the major difficulties in the piece. for example, a double trill at the right hand is required around the last part of the finale of his last piano sonata.
It seems to me that the trill has become one single note with a kind of texture which is different from two oscillating notes. and Beethoven was in some preference with that kind of texture.