Although preliminary grading is carried out on green lumber for the purposes of monitoring yield and inventory, the final grade of each board is usually determined after drying. All sawn lumber is inspected and graded to the rules of the National Hardwood Lumber Association (NHLA), details of which are published in their Rule Book and summarised together with visual examples in the AHEC publication, 'An Illustrated Guide to Hardwood Lumber Grades'.
These rules were first established by the lumber industry over 100 years ago to service the American domestic furniture trade, and are nationally and internationally accepted. They are used as a basis for export, and are widely acknowledged as the most consistent grading standards for temperate hardwoods anywhere in the world. This is in contrast to Europe where many national and regional approaches require a greater element of buyer inspection.
The NHLA rules consist of lumber grades which are determined by visual inspection based on yields of clear (free of defect) cutting areas. They are provable by mathematical calculation. The official grades set out minimum requirements, however industry interpretation does vary on upper limits. Many individual exporters have established modified rules for specific export markets. For example, Comsels grade, and the allowance of a small percentage of 4" (101.6mm) and 5" (127mm) widths and 6' (1.83m) and 7' (2.13m) lengths in FAS 'prime' parcels. In the United States special grading rules were developed many years ago for walnut and butternut, full details of which are included in the rules for the measurement and inspection of hardwood available from the NHLA.