To Play Parallel to the Bridge or Not?
By Hans Jørgen Jensen
I wrote this article several years ago and tried to describe how our knowledge about sound production of the string instruments had evolved over time. It is still incredible to me how the string world at first would not accept the discoveries by Dr. F. A Steinhausen (1903) about the physiology and biomechanics of sound production on a string instrument.
When Percival Hodgson in 1934 published the book Motion Study and Violin Bowing (where he showed that bowing motions were generally curved and elliptical and in no case did the movement remain absolutely parallel with the bridge) a fierce polemic took place in The Musical Times where people argued against his discoveries.
It took more than 40 years before experts of string playing accepted the discoveries of Dr. Steinhausen, Wilhelm Tredelenburg, and Percival Hodgson.
A fascinating question to ask is how it was possible for all the great string players before the 20th century to play great without totally understanding the theories about sound production. The answer to that question is easy: Most of what we do is done intuitively and even thinking about what we do makes it more complicated. Also, the rule about playing parallel to the bridge is still an important fundamental rule and changing the contact point where the bow is not parallel to the bridge is mostly done intuitively.
It is, however, very helpful for string players to know that when changing the contact point, the bow should be slanted and the faster the bow is changing the contact point, the more the bow should be allowed to slant.
Here are a few rules from Chapter 20 in PracticeMind: The Complete Practice Model:
On down bows: the bow will move in the direction of where the tip is pointed.
On up bows: The bow will move in the direction of where the frog is pointed.
To play parallel to the bridge or not that is the question
To play parallel or not? You might not realize it, but this is a very controversial topic.
To play parallel to the bridge is an aspect of string playing that has often been misunderstood – one that results in some heated discussions among teachers and performers of stringed instruments even today.
The five human senses are amazing and the way each of us perceives and experiences the world in unique ways is one of the wonders of being a human. When discussing sound production on string instruments, there is no question that there are many successful ways of playing with a beautiful sound. It is also important to note that we all have different ideas and images in our minds of the ideal sound on a violin, bass, viola, and cello. I am in no way trying to tell anybody what to like or not like, or what to do or not do. However, I am very eager to try to ignite some excitement into this topic!
Perhaps we should first take a journey into the past to see what the great string minds of the past thought about this topic:
The Intention of Music is not only to please the ear, but to express sentiments, strike the imagination, affect the mind and command the passions. The art of playing the violin consists in giving that Instrument a tone that shall in a manner rival the most perfect human voice, and in executing every piece with exactness, propriety, and delicacy of expression according to the true intention of music. 
Francesco Geminiani wrote these inspiring words in the introduction to his treatise The Art of Playing on the Violin. In this introduction, Geminiani also said:
In playing long notes, where the bow is drawn from one end of it to the other. The bow must always be drawn parallel to the bridge. 
36 years later Leopold Mozart said:
The student should not move the bow back and forth between the fingerboard and the bridge. Or use a crooked bowing. But should stay on the string not too far from the bridge, and there in a patient manner look to draw a good and pure tone from the violin. 
Other famous violin and cello pedagogues over the years (G. Loehlein-Baillot-J.L. Duport-Spoor-Joackim-Moser- Capet-Auer and Flesch) all agreed that the bow should move in a straight line parallel with the bridge. [3a] [3b]
The first person to challenge this concept was the German scientist, Dr. F. A Steinhausen, who wrote a book on the physiology of bowing that was published in 1903.  Steinhausen occupies a very important role in the history of instrumental playing because he was the first physician who investigated string playing from the viewpoint of motion-physiology and biomechanics. He pointed out in his book the lack of supporting evidence for the rules described in all the well-known method books. He also clearly felt that the great players never truly follow or pay much attention to those rules.
Another important person in the history of science and string playing was Wilhelm Tredelenburg, who in his book published in 1925 commented on skewness in bowing in double bass playing.  He described the effect that skewness in bowing results in a drift of the bow-string contact point along the string under the influence of the stick-slip interaction. 
Later in 1934, Percival Hodgson did extensive observations by recording bow motions with cyclegraphs. Hodgson literally took thousands of cyclegraphs, from every conceivable aspect and of every kind of bowing: “In no case did the movement remain absolutely parallel with the bridge, and the crookedness always fell into definite types”. He also observed that bowing motions were generally curved and elliptical, which in his opinion was necessary for making smooth bow changes.  The publication of Hodgson’s book led to a fierce polemic in The Musical Times. 
At present most experts on string playing agree on the concept that bowing trajectories are very often curved. A number of method books advocate the slanted bow as a tool for changing the contact point.    
When discussing the issues of the slanted bow or playing parallel to the bridge or not we need to look at the biomechanics of the bow arm and the physics of the vibrating string. As part of this series, a number of articles about the biomechanics of the bow arm will be presented. In my opinion, it is very surprising that very few research experiments have been done on the topic of playing parallel to the bridge. However, a brilliant young scientist Erwin Schoonderwaldt has performed several groundbreaking experiments on a number of topics related to string playing and the vibrating string, including On the Use of Skewness in Violin Bowing: Should the Bow be Straight or Not? In my second article in this series, I introduce Erwin Schoonderwaldt to the readers of String Visions and discuss his experiment on skewness in violin bowing.
 The Art of Playing on the Violin. Francesco Geminiani London 1751.
 Translated from German to English by Hans J. Jensen from: The Violin School by Leopold Mozart Published 1787.
[3a] Senso-Motor study and its application to violin playing. Dr. Frederick F. Polnauer and Dr. Morton Marks. American String Teachers Association, Urbana,Illinois 1964.
[3b] Essay on the Fingering of the Violoncello and on the conduct of the bow. Jean-Louis Duport, Augener & Co. London 1806.
 Die Physiologie der bogenführung auf den streichstrumenten, von dr. Friedrich Adolf Steinhausen. Leipzig Breitkopf & Härtel 1903.
 Die Natürlichen Grundlagen der Kunst Des Streichinstrumentspiel von Wilhelm Tredelenburg. Berlin Verlag Von Julius Springer 1925.
 Translated by Erwin Schoonderwaldt from Die Natürlichen Grundlagen der Kunst Des Streichinstrumentspiels. On the use of Skewness in Violin bowing: Should the bow be straight or not. Acta Acustica United with Acustica Vol. 96 (2010) 593-602
 Motion Study and Violin Bowing. Percival Hodgson. J.H. Lavender & CO.,London 1934.
 On the use of Skewness in Violin bowing: Should the bow be straight or not. Acta Acustica United with Acustica Vol. 96 (2010) 593-602
 Essay on the Craft of Cello-Playing Volume I. Christopher Bunting. Cambridge University Press 1982.
 A Guide to Advanced Modern Double Bass Playing. Knut Guettler. Yorke Edition Thornhill Square,London N1 1BQ, England 1990.
 The Art of Bowing Practise by Robert Gerle. Stainer & Bell, London 1990.
 Orchestral Bowing Style and Function. James Kjelland. Alfred Music Publishing 2004.