Bow Stroke Basics
The bow should move parallel to the bridge of the instrument. Another way to think of this is the bow should be perpendicular to the string. This simple rule should be easy to follow, but it is actually quite challenging and requires much practice, as well as proper guidance in the early stages of instruction.
There are a few reasons why it is difficult for string players to move their bows parallel to the bridge. First, from the player's perspective, it is difficult to make a correct judgement about whether the bow is parallel to the bridge - the bow appears to be parallel when in fact it is not! This is why teachers encourage their students to practice in front of a mirror. If you look into the mirror while you draw the bow, it offers an eye-opening perspective on the angle of the bow to the string. Some teachers use "string guides" mounted on the instrument to help students develop the correct bow path.
Second, when the player crosses strings with the bow, a bow angle adjustment is needed to keep the bow perpendicular to the string. A common problem for double bassists is crossing from the D string to the G string. Without an adjustment when crossing, the bow will not be perpendicular to the G string, and will make a scratchy sound.
Third, as the player moves the bow from frog to tip, a wrist and finger motion adjustment will need to be made to keep the bow moving perpendicular to the string. This motion varies among violin, viola, cello and bass, and from string to string on each of these instruments, and will be discussed in depth later in this tutorial.