Four Books on Human Proportion

Following this, we arrive at the measurement of the arm original: brachij mensuram, which we have set down as follows:

From the shoulder original: humero—specifically where the head of the humerus connects to the collarbone original: jugulum—down to the elbow original: cubitum measures two units of 11.

Furthermore, from the shoulder to the lowest point of the muscles is one unit of 10.

From the elbow to the very tips of the fingers original: digitos is one unit of 4.

From the fingertips back to the end of the hand original: extremam manum; the wrist is one unit of 10. However, it is not ungraceful original: inconcinnum if one prefers to extend this to one unit of 9, should that by chance please the artist.

The diagram illustrates a right-angled triangle labeled a-b-c. A series of lines radiate from point a to the vertical axis, intersecting a diagonal line representing the "Equator" or the central proportional divider. This geometric method allows the artist to maintain consistent ratios even when the scale of the drawing changes.