Pianists have it easy. They couldn't play out of tune if they tried. When I was "taking" violin from Mrs. Zacharias back in the '40s [the 1940's, that is], I had the odd notion that playing in tune was like having naturally curly hair -- something you were born with. It was not until half a century later, when I was studying the viola seriously with Delores Thayer, that I discovered one could listen to oneself and adjust the finger on the string to approach closer to being in tune. It was a revelation. But the real source of the string player's resentment of pianists is doublestops.
A doublestop is the playing of two notes at the same time. On the piano, this is child's play -- literally. Chopsticks, that standard of childhood, is just a series of double stops, the two notes played with the two forefingers. Anyone can do it, and it even sounds sort of nice. On the viola, matters are entirely different. The notes of a doublestop are played on two different strings by two fingers of the left hand, the bow being drawn over both strings at once, to produce a chord. This is a very great deal harder than it looks. After all, it takes several years of serious work just to make a sound on the viola that does not drive everyone else out of the room. A scale is a triumph. As a boy, I was completely intimidated by doublestops and rarely attempted them. But Delores Thayer, or Loree, as she is called, was having none of that, and put me through an entire book of exercises devoted to doublestops. As a consequence, I have reached the point at which I can, in the course of playing a line of music, actually include the occasional doublestop without blanching or causing others to cringe.
Which brings me to the Andante Cantabile of Mozart's beautiful violin/viola duet, K 424, which I am currently practicing in preparation for a session with a local amateur violinist on Monday. The first two movements have a number of manageable doublestops, but the viola part of the third movement, the Andante Cantabile, is six lines long, five lines of which are nothing but doublestops! On my beautiful viola, these have a rich, sonorous sound, if I can play them! But it is stretching my abilities to the limit to get through the movement at all, and doing so in a manner that provides the beautiful underpinning for the violin's obligati is the hardest damned thing I have ever attempted.
Molzart played the viola, and I am sure such things posed no problem for him at all. He probably played the movement holding the viola upside down as a parlor trick, at least if Amadeus can be relied upon.