Hence dead branches produce knots which are nothing more than pegs in a hole, and likely to drop out after the tree has been sawed into lumber.

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.[32] [Footnote 32: See Standard classification of structural timber.
Yearbook Am.Soc.for Testing Materials, 1913, pp.

300-303.
Contains three plates showing standard defects.] Knots materially affect checking and warping, ease in working, and cleavability of timber.

They are defects which weaken timber and depreciate its value for structural purposes where strength is an important consideration.

The weakening effect is much more serious where timber is subjected to bending and tension than where under compression.