What is the Canon of proportions?

Answer 1

It is the canon law, to which Egyptian artist were mandated to regularize dimensions and scale. A guideline for Egyptian artist to follow.

Added 10/03/09: According to the law, the human body (as depicted in their art) must be 18 units length to the hair lines and just under 19 to the top of the head. The bellybutton should be at 11 units length. This is beginning the measurement from the soles of the feet.

Basically, you take 19 equal-in-high units and you draw the hair line at number 18 and bake sure the bellybutton ends up in the 11th unit.

During this period (as stated by the previous answer), it was literally mandated for artists to follow these rules (freedom anyone?). As known to artists, a canon is the basic model or rule that characterizes a style.