Teachers, beware!  Help the student understand the sensation of pulling a straight bow!  We most often see problems on the A string because of its extreme angle.  To discover this angle, check out Defining Planes.  The most common error is the student pulling the arm backwards while pulling the down bow, keeping the elbow bent after the halfway point in the bow.  The solution  requires pushing the hand forward and out – allowing the lower arm to extend from the elbow. After having explored this issue with a student MANY times, yesterday I took a quick snapshot of his bow angle on the A string.  His response was, “oh, wow”!  Yesterday, one picture was worth a thousand words and the student quickly repositioned his bow consistently perpendicular to the A string.  Ahhhh, much better!!!!