I recently created a video delving into the topic of double stops, and why it can be so gosh darn difficult to get them right. There are a number of factors that play into their difficulty, but today, we will be focusing on one of them: the left hand.
If you've ever found yourself playing a tune while trying to incorporate an open string (drone) along with the melody string, and discovered a less-than-pleasant sound emitting from your violin, you know what I am talking about here. It is very common to be bowing two strings at once while the left hand is supposed to be on just one of them, but you find yourself having trouble avoiding the second string.
Let's say you are playing 'Cripple Creek', which starts with a shuffle, then fingered melody notes on the E string while continuing to bow on both the A and E strings simultaneously. You get past the shuffle, place your third finger down on the E string, and notice not a beautiful, resonant chord, but rather, a squawking cry for mercy from your fiddle. This is likely due to your third finger getting a little too comfortable, and entering the A string's territory, thus producing these unanticipated and unappreciated sounds.
So what can you do? Firstly, it's important that you make sure your left hand technique is optimal. No crooked or collapsed wrists, no flattened fingers - nothing that could be compromising the quality of the notes, intonation, or in this case, double stops.
Next, you'll want to create space between your finger and the A string. It may sound obvious, but this is something you'll want to check before going any further.
If you think there is enough space between your finger and the string but you still find the finger accidentally bumping the open string when attempting double stops, you'll want to check the angle in which the finger is placed on the string, and....
the ELBOW. This is possibly the most important, yet somehow often overlooked aspect of violin playing in general, but especially when it comes to playing double stops. Where is your left elbow positioned underneath the fiddle? If you are trying to avoid the lower string, try pulling your elbow back a bit. If it's the higher string you're trying to avoid (say your left hand is playing melody on the A string and you're droning the open E string), perhaps bring your elbow forward.
As you're mindfully connecting with your elbow and harnessing the energy of fiddle players past to find that perfect angle, continue to keep an eye on your left hand and wrist. Ensure that as you are repositioning your elbow, your wrist, hand, and arm are all moving together as one, beautifully aligned and ultra relaxed unit.
To put this all into practice before jumping back into the tune, I'd recommend playing scales alongside a drone string to make sure you really have this concept down pat. You could start with your bow on both the A and E strings, your left hand on the A string, and play A-B-C#-D nice and slowly, while checking in with your elbow and the space between the E string and each of your fingers as you place them down.
Then, to try the opposite, you'd place your bow on both the D and A strings, and repeat the process, while making sure there is adequate space between your fingers and the D string, now.
And there you have it! Your left-hand-accidentally-hitting-other-strings-during-double-stops problem, solved. Keep in mind that as with anything, just because you now have the tools for this job, doesn't mean everything will suddenly fall into place right away. It may take a little practice and persistence to get the hang of using your elbow to guide your hand/fingers in the right direction, but you will get there, and once you do, it will have been worth the extra effort!