Wood as a Building Material - Carpenter's and Builder's Math, Plans, and Specifications - page 58

(A) LOOSE KNOT
(B) SOUND KNOT
Figure 2-12 Black or loose knots (A) are caused by broken
branches in the log. Bulging layers of growth cause a large swirl
of cross grain at the surface of the board surrounding the knot.
Tight red knots (B) are caused by a live branch in the log. The
knot will not fall out and grain is relatively straight around the
knot. (Courtesy Practical Restoration Reports)
SPLITS
MEDULLARY RAYS
Figure 2-13 A board with splits along the medullary rays. This
condition is caused by too-rapid kiln-drying.