In grading lumber and structural timber, knots are classified according to their form, size, soundness, and the firmness with which they are held in place. This firmness is affected by, among other factors, the length of time for which the branch was dead while the attaching stem continued to grow.
Is there an answer to this question (If it cannot be answered, say "unanswerable"): Along with firmness, soundness, and form, what property of a knot is classified?
size