Many violinists, violists and cellists have experienced the joys of playing a concerto with an orchestral reduction which has been written for the piano. Often, this is a more fulfilling experience for the string player than for the pianist. Part of the reason is because the reduction of a complete orchestral score to a piano part inevitably leads to ‘un pianistic writing’, but another reason is that tremolos (or tremolandos) are exceedingly hard to play on a keyboard instrument. They are sometimes written for virtuosic effect but the rapid depression of a single key is not practical, therefore the notation changes the tremolo to a rapid oscillation between two pitches an octave apart.
In reverse, there are many phrases that sit comfortably under a pianists hands that might be incredibly awkward when transferred to a stringed instrument which is tuned in fifths. An example of this could be rapid, slurred semiquavers that occasionally jump from an upper string to say, two strings down. This may fall within an octave hand span, yet the effect of rapidly jumping two strings could be clumsy and disrupt the flow of the music, even though it’s technically playable.
As many of the great composers knew, writing for stringed instruments well often involves ‘open strings‘ and writing within the key signatures which naturally suit the instruments. Any key is possible but as an example, the key of D major would project better than that of Db major. The reason for this is that there are more resonating notes in D major than in Db. To give a brief explanation, when played perfectly in tune, any G, D, A or E on the violin can be made to ‘ring’ and resonate more than other notes. As an example, the note A in the first position on the E string is an octave above the open A string and when played in tune can be made to ‘ring’ with the other string in sympathy. The same is true of the note A on the D string (the same pitch as the open A string) which is a very strong note on the violin with it’s ability to resonate with the open string and therefore a really meaty and rich sound can be produced on this note. On the piano, most notes in the middle register of the instrument have more or less an equal tendency to resonate (although I am sure there are many subtleties and differences between them). The ruling principle is not necessarily which key signatures sound strongest but which are easiest to play in terms of hand position. The point is, what sounds easy and right on a keyboard is completely different to what sounds natural and best on a stringed instrument.
When writing for strings (e.g a string quartet), the importance of using counterpoint (where each instrument has its own independent melody line that enhances and complements the others) is very possible on a piano, but for those writing chords it’s essential that each instrument in the string section has a line of melody which could be played in it’s own right and still sound musical. When writing chords, which notes are assigned to which instrument (whether violin, viola, cello or double bass) can make a huge difference to the overall sound and flow of the track.