In the U.S. and Canada, visual inspection for edge knots is done in conjunction with both technologies. Visual lumber grades are based on complex sorting criteria, which typically include the assessment of knots, the slope of grain, checks, shake and splits, density, decay, heartwood versus sapwood, pitch pockets and wane. Knots are evaluated by size and position and whether they are loose or tight. Cross grain within a board is greatly restricted because it has a dramatic influence on many strength properties. Checks, splits and shake affect strength in shear.
Density is often a good surrogate for mechanical strength tests, but it can be misleading.
Decay is generally prohibited, although some pocket rot is allowed. Heartwood and sapwood have similar strength properties and, therefore, are not distinguished for stress grading, although their presence may be noted for species in which the heartwood has much higher decay resistance than the sapwood. If the lumber is to be pressure treated with preservative, sapwood is preferred since it absorbs preservatives more readily. Pitch pockets can generally be ignored if they are small and limited in number. Straightness is undoubtedly the most important criterion, with good reason.