Just as most shifts between single notes with different fingers use a guide finger, double stops also use guide fingers and the same basic principles apply.
For instance in the first shift here do the shift on the 'old' fingers, that is the previously placed 1st and 3rd fingers.
Of course in performance this is more like an interchange of fingers, where we would start the shift on the 2nd and 4th fingers and then the 1st and 3rd fingers take over.
When shifting from one pair of strings to the next pair we choose just the finger on the middle string to travel on. So for the second shift we practice travelling on the 2nd finger from 3rd position down to 1st position.
Make sure that you also use guide fingers when descending. For instance in the first descending shift, practice travelling down on the 1st and 3rd fingers. Then to cross back to the D and G strings, travel up on the 3rd finger.
SEE
Bibliography
- for The Art of Practising the Violin by Robert Gerle
- and The Joy of Shifting and Double Stops by Jerrie Lucktenberg
Scores
- 3rds in C Major - Guide fingers - violin
- 3rds in F Major - Guide fingers - viola
...