Face length is a very genral approximation of a hand size used when teaching figure drawing students about human proportions. Lot of new students will make  sizeable proportion errors while learning... and make hands and feet twice or half the size they are on the average. To quickly and relatively simply get them back in a ballpark, they are often prompted to compare lengths of hands and feet to head and face. These things are loose guidelines to prevent gross errors in figure proportions, and not something meant to be very exact.

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Renderosity Store  Personal nick: Conniekat8
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