If Dia. [XI] is cut from a piece of stiff paper, and the edges of the square are connected, and that circle formed in one continuous shape, like that of a lamp shade, it will form a slope of 30 deg., or near that slope, and corresponds to all diagrams, with a square of 17½, and with the plumb line base in front, from which base the shoulder slope is 30 deg., as shown in Dia. [IX].
Here it will be noticed that the bases, as adopted in this work, mean something more than imaginary lines. They mean actual slopes of the human form, and quantities which are parts of the square or compass, and which instruments are known and used by all civilized nations.
Squares of Seventeen and a Half and Twenty.
The squares of 17½ and 20, and lines 9 and 11¼, are both permanent bases. But the lines which connect the back to the foreparts, or to the side piece, or join lines 9 and 11¼ over the front, require especial explanation. Line 9 on the front base of the angle of 135 deg., and line 11¼ on the front plumbline, meet in the center of the square of 20 and in the position as indicated on Dia. [II]. Line 9 is broken between the side-piece and back, and turned 15 deg. upward, but will be of the same space as the square of 20 when the side-piece and back is connected. The square of 17½ is one-eighth less than the square of 20, and harmonizes with the division of the circle, as shown on Dia. [XII].
In both Dia. [XI] and [XII], the circle has a full diameter of 40 numbers, and the triangle as shown in Dia. [XII], has 35 numbers on each line, making an equi-lateral triangle. One-half of the circle is used as a square of 20, as in Dia. [XI]. One-half of the triangle is used as a square of 17½ as in Dia. [XII]. The square of 20 represents the amount used for the whole half vest, seam and surplus included, and the square of 17½ represents the half breast measure. The correct combination of the square of 20 and 17½, can only be used on a vest with one seam in the center of the back, and one seam under the arm. On coats the square is enlarged on account of more seams and other reasons elsewhere explained; but for all that, the squares of 20 and 17½ are a true guide for all coats. It will be seen that the squares of 20 and 17½, as used in this book, are not imaginary lines, but based upon a scientific calculation from the center of a circle or the corner of a square, which are used by the whole civilized world as points, or fixed facts.
By further examination of Dia. [XI] and [XII], it will be found that all the essential points necessary in cutting a vest can be found within a circle, and without using a scale—all of which is further explained in the article entitled, “[Scientific Calculation].” Another reason why lines 9 and 11¼ on the front bases are adopted, is the following: Lines 9 and 11¼ meet in the center of the square of 20. Line 9 runs at right angles with the center of the front. Line 14 on the back runs at right angles with the center of back. When the garment is upon the body, both run in the same direction, are in their natural positions, and on a square of 20.