For the past while, I've been learning Beethoven's Sonata no.3 (op.2. no.3) and just coincidentally, because I had learned - in a so-so manner- the first movement Waldstein sonata in the past, (like 20 years ago) I started playing it as well and to make a long story short, I decided to learn the entire Waldstein sonata, alternating with the no.3 (practising each on alternate days) and I discovered that the learning of both is complementary because they share many of the same motifs - they are both nominally in C major- and without placing the scores side-by-side to specifically delineate their similarities, it is obvious to me that I have stumbled upon a combination which makes the learning of both, easier! I wonder what other combinations of pieces (Beethoven sonatas or otherwise), share this complementarity.