Born some time around the early 4th century B.C., Siddhārtha Gautama was a Nepalese monk and wandering sage whose teachings went on to underpin Buddhism. As Buddhism spread, pictorial representations Gautama —the Buddha—were expected to be so particular that guidelines emerged as to how he should be drawn. Public Domain Review points to a book from the 1700s that shows, precisely, how the Buddha and other important Buddhist figures should appear.
Written in Newari script with Tibetan numerals, the book was apparently produced in Nepal for use in Tibet. The concept of the ‘ideal image’ of the Buddha emerged during the Golden Age of Gupta rule, from the 4th to 6th century. As well as the proportions, other aspects of the depiction – such as number of teeth, colour of eyes, direction of hairs – became very important.
The iconography of the Buddha is highly specific. Representations of him should abide by 32 greater Laksanas and 80 lesser ones—particular features and bodily markings that are important components of the Buddha's image, says Ohio State University. For instance, according to the Victoria and Albert Museum, the Buddha should be seen with wheels on his hands and the soles of his feet. The Buddha should have webbed fingers and toes; long arms; soft, curly hair; and “eyes the colour of sapphire,” among other distinctive features.
H/T Kyle Hill