Why do so many pieces in the violin repertoire have a sharp key signature? Most of the great violin concertos are in the key of D (Beethoven, Brahms, Tchaikovsky, Sibelius, Prokofiev, etc.) and this is no coincidence. The keys of G, D, and A use the open strings of the violin, which have much more resonance than any of the ‘stopped’ notes, thereby giving the instrument a greater projection and brilliance. There is also the fact that the natural shape of the hand falls comfortably (if starting on the d string) d, e f#.
When playing in flat keys, such as Ab, Eb, Bb, there is less opportunity to use the open strings of the violin and thus the natural resonance of the instrument. The great violinist and composer Nicolo Paganini was only too aware of the importance of key. In his 1st violin concerto he scored the solo violin part in D major, yet writes the orchestral parts in Eb major. The solo violin is tuned up a semitone in order to match the pitch of the orchestra, though the use of D major creates a more brilliant and open sound for the soloist.
So next time anyone tells you that open strings are ‘too easy’ or that advanced players only use high positions, remember that by using the open strings of your violin you are employing the full resonance of the instrument!