After practicing Dounis Opus 12, which, to my astonishment is all double stops, I finally figured out something practical and truthful about playing double stops: the fingers are the easy part. My "it's the bow, stupid" realization came to me when I was tuning this morning. I realized that as string players we spend at least 90% of our time (if not more--unless, like me, you do an hour of Dounis) playing on one of four single strings. When we play single notes on single strings, we pay attention to the straightness of our bows, our bow changes, the contact of our bow to the string, the loudness and softness, and we can usually have a pretty good shot at sounding good on a series of given notes if all goes well and we are paying attention. At least it works for me.
So, while I was tuning today I realized that if I wanted to be able to hear both of my open strings for the entire length of a bow (at a slow tempo), I had to really pay attention to having contact on both strings at the same time for the whole bow stroke. I found it rather difficult to do, so I did it for a long enough time to feel like I could keep both strings equal. After my bow stroke was under control, I found it far easier to really tune my strings.
When we play fifths, whether they are open or stopped, we are dealing with exactly the same length of string for both notes, so the bow shouldn't have to do much compensating. But once in a while, if the bow isn't exactly straight, or if the contact with both strings is not exactly equal, or if the strings are out of tune, the exciting magical resonaces that happen when playing fifths seem to "fight" with one another for rank in the overtone series.
When we play thirds there is a two-inch (or more in the case of minor thirds) difference between the length of string from note on the lower string to the bridge, and from the note on the upper string to the bridge. Our bow are has to take those different tensions into account, not to mention all the other sympathetic resonances that come up when the third is in tune. It is like planning a quiet picnic for two and finding that the whole family has come along. Sixths are far easier than thirds because we only have an inch or so (less with minor sixths) difference in distance from the bridge. The overtones that pop up are rich and comforting because the interval is usually easy to get in tune, and the bow doesn't have to stress to keep either the pitches or the overtones under control.
An octave is only a little more difficult for the right hand than a sixth because if an octave is in tune, all you hear are the two notes of the octave. There is a three-inch difference of string length between each finger and the bridge, and it is easy to forget to listen for both notes all the time.
Dounis has several exercises using fourths. The fourth combines problems with the fifth (a fourth is just a fifth turned upside down, so all the sympathetic resonances of the fifth are there, vying for position) and the difference in string length of the third. They are also hard to hear and hard to find when you have to shift to one. They are also really annoying to listen to.
I have decided the the difference between Dounis and Sevcik is that Sevcik exercises can actually sound beautiful, and it can be downright relaxing to practice them. Dounis' Opus 12 exercises do not sound beautiful, and I always have to be on my toes when I practice them. In the past nine days I have noticed that when I practice music my sound is far more consistent, my double stops are better in tune, and I have a lot more bow control. And everything transfers remarkably well to the viola, particulary my new found strength in my third and fourth fingers.
I'll say one more thing, and then I'll stop: I have always thought that Sevcik wrote his books of exercises so that violinists could acquire the technique necessary to play Dvorak violin parts, and my silly reason for the reason the Beethoven "Kreutzer" Sonata is called the "Kreutzer" Sonata? Because in order to play it you have to have mastered all the Kreutzer etudes.