The part which is nearest to the earth... is without knots and is "clear." But the upper part, on account of the great heat in it throws up branches into the air through the knots and this... is called "knotwood" because of its hardness and knottiness. The lowest part, after the tree is cut down and the sapwood of the same thrown away, is split up into four pieces and prepared for joiner's work, and so is called clearstock.

Chapter IX, Sec. 7 - De architectura (The Ten Books On Architecture) (~ 15BC) - Book II