A Treatise on Painting/Chapter 54
Chap. LIV.—How near behind the Back one Arm can be brought to the other, Plate III. and IV.
When the arms are carried behind the back, the elbows can never be brought nearer than the length from the elbow to the end of the longest finger; so that the fingers will not be seen beyond the elbows, and in that situation, the arms with the shoulders form a perfect square. The greatest extension of the arm across the chest is, when the elbow comes over the pit of the stomach; the elbow and the shoulder in this position, will form an equilateral triangle.