The four strings of the violin are tuned in fifths. Thus, it is necessary for the violinist to depress two strings at once with the same finger in order to play a perfect fifth (except on open strings).
However, using one finger to play two strings at once is very difficult. This is because the surface area of the fingertip that comes in contact with the fingerboard is simply not wide enough to cover both strings.
When confronted with a perfect fifth, it may be tempting to simply roll the hand under the violin to cover both notes with a flat finger. However, for several reasons, playing with flat fingers creates tension in the left hand. First, more pressure is required to depress the string because a flat finger exerts fewer pounds per square inch on the fingerboard than one standing on its tip. Second, the wrist must deviate uncomfortably to the left rather than remain straight. Finally, it takes time away from the player to shift the hand away from and back to its ideal position. As a result, he or she may feel rushed and react by squeezing the hand against the fingerboard. Often, a violinist will continue to play with flattened fingers even after the perfect fifth has come and gone– simply because, when tense, the hand tends to press against the underside of the fingerboard and does not like to let go.
How should a violinist play a perfect fifth without using a flat finger? It is best to find a position for the fingertip in between the two strings that catches both at the same time. Because every student’s hand and finger shape is different, it is usually necessary to experiment to figure out the optimal placement for each finger. In any event, it is far better to create a slightly imperfect fifth than to modify the entire hand position to accommodate this interval.