To Play Parallel or Not: A Controversy of Technique, Part 2
By Hans Jørgen Jensen
Continuing the epic question: to play parallel or not?
Sad to say, there is not enough communication between musicians and scientists on string acoustics. These scientists can offer us an incredible amount of knowledge and understanding. All we have to do is ask.
In the second article of this series, I would like to introduce a brilliant young scientist from Sweden Erwin Schoonderwaldt who received his PhD from the Department of Speech, Music and Hearing at the Royal Institute of Technology in Stockholm, Sweden, and has completed several important studies on string acoustics including his PhD thesis “Mechanics and acoustics of violin bowing.”
In this article I would like to talk about and quote from Dr. Schoonderwaldt’s scientific paper “On the use of Skewness in Violin Bowing: Should the bow be straight or not?” 
Fig. 1 from "On the Use of Skewness in Violin Bowing: Should the Bow be Straight or Not?".
I highly recommend that you all spend some quality time studying this wonderful paper so that you really understand the science behind this topic.
Before getting started I would like to tell an anecdote from my days as a student:
As a young cello student I was always taught to play parallel to the bridge. After returning from a concert featuring the French cellist Paul Tortelier, I said to my teacher, “The concert was fantastic; such a wonderful sound full of expression and colors…but Mr. Tortellier hardly ever played parallel to the bridge.”
My teacher responded, “Tortellier is so good that he can get away with it.”
His answer was very surprising to me, and it wasn’t until I became a teacher myself that I fully realized why he had given me this answer. I think my teacher knew that advanced string players hardly ever play parallel to the bridge. He, however, wanted to stress the importance of what playing parallel to the bridge felt like so that one may truly develop the feeling for the vibrating string.
Here I will quote some of the important sections from Dr. Schoonderwaldt’s paper. (As stated above you must read and study the whole paper in order to really understand this topic).
Bowing parallel to the bridge is by many players considered as the golden standard. However, in practice straight bow strokes are rarely observed, and the bow can be considerably slanted even in the performance of renowned players. In any case, the angle of the bow with the violin (skewness) is likely to form an important control parameter, which has hardly been addressed in scientific studies of violin performance. In the current study measurements of skewness in violin and viola performance are presented, and possible explanations of the observed behavior are offered. The results provide strong indications that skewness fulfills an important function in controlling bow-bridge distance, integrated in players’ performance strategies.
During the last two decades, several studies of violin bowing in performance related to the control of the sound have been conducted using sensor systems, motion capture techniques or combinations of those. In these studies the focus has been on the main bowing parameters at the bow-string contact with a direct influence on the string vibrations, in particular bow velocity and acceleration, relative bow-bridge distance and bow force. These bowing parameters are well established in bowed-string theory and simulations dealing with playability….
As yet, scientific studies of the use of the bow angles are scare, and not much is known about their function in bowed-string performance. A common opinion expressed in classical pedagogical works is that skewness of the bow inevitably leads to inferior sound quality. However in practice the bow is only rarely drawn exactly perpendicular to the string, without noticeably influencing the quality of the tone.
Aims of the Study
The major goal of this study is to clarify the use of skewness in violin and viola playing, and to identify possible factors on which it depends. This is done using empirical data from a performance study with advanced violin and viola players, whose bowing actions were recorded using a method developed by Schoonderwaldt and Demoucron. It could be hypothesized that skewness mainly has a secondary control function,i.e., that it is used by players to exert control of intentional changes in one or more main bowing parameters, without having a significant direct influence on the sound quality by itself. In particular, the possibility that players use skewness in a functional way to control bow-bridge distance is investigated The mechanism for the natural drift of the bow-string contact under influence of skewness will be explained. An analysis of the use of skewness in performances of crescendo and diminuendo bow strokes will be presented, demonstrating the role of skewness in the control of bow-bridge distance.
A number of evidence were found in the analyses, that support the notion that skewness was used by the players to control bow-bridge distance.
A simplified mechanism was presented for the natural drift of the bow-string contact point along the string when the string is bowed at an angle, and an equation was derived to calculate the drift velocity as a function of skewness and bow velocity…. Skewness can therefore be considered as a secondary bowing parameter, playing an important role in the interaction between the player and the instrument.
Given this function of skewness in playing it is hard to maintain the position that the bow by rule should be parallel to the bridge. This does not imply that a good player should not be able to perform straight bow strokes. However, it is probably most important that the player is able to exert a conscious control of skewness depending on the musical demands during performance.
As I stated in the last article controlling sound production and how to navigate the bow on the string is at the highest level mostly controlled by instinct. However, when learning a new solo or chamber music work it is extremely important to spend some time organizing the bow strokes.
Also, in spite of this article being about the slanted bow stroke I still want to remind you that practicing son filé with the bow being parallel to the bridge is still one of the most helpful exercises to practice. Son filé (spinning tone or the spun tone) was advocated by many of the great violin pedagogues of the past including Carl Flesh in: “ The Art of Violin Playing” and Ivan Galamian in: Principles of Violin Playing and Teaching.
Here is the complete article: