When a written part consists of C-major scale, the first of these instruments will render it as an A-major scale, and the second as a B-flat-major scale. In fact, two different versions, with different transpositions, are common today: one in A, and one in B-flat. The clarinet is more complicated, because it is a transposing instrument. Chopin is said to have taught his students the B-major scale before any other. The B-major and D-flat major scales are a better fit to the shape of the hand, because the fingers cover the black notes of the keyboard, and the thumb has room to cross underneath and reach the white notes. C major is conceptually simple and very familiar, but it forces the hand into a relatively flat position since all the diatonic notes lie on a single plane, so thumb-crossing is a challenge, and one can't easily play with the hands interlocked. But the difficulty should not be overstated: a lot of string repertoire is highly chromatic, so even a piece in a relatively comfortable key such as F major can roam through a region based on D-flat. The key of D-flat forces string players to refer to potentially less-familiar fingerings and hand positions. For string players (violin family), their instruments have open strings of G/D/A plus either C or E, so the most familiar fingering patterns are the ones based on scales that include most or all of these open-string notes. The 'difficult' key signatures vary by instrument.

I'm curious: Why is having many sharps or flats difficult, is it technically difficult? Would difficulties arise also in Clarinet playing for instance?

Then I read about 12-tone elements and that explained everything My initial thoughts were: 'Intellectual chaos, modern chaos and difficult'. Karl recommended me SQ 12 of Shostakovich in another thread. Quote from: Tapio Dmitriyevich Shostakovich on December 17, 2011, 08:39:13 AM

D flat major key signature