Wood Flooring Grades measure hardwood planks on a scale of ABCDEF. Each grade has associated characteristics that are taken into account, such as size of knots, sap content, color uniformity, and more. The grades are as follows: Prime or AB Grade AB Grade is the highest grade of wood flooring possible. The wooden planks at this grade are cut from the center of the log, offering very few and almost unnoticeable/unobtrusive knots. Wood flooring graded as an AB boasts the most uniform appearance due to minimal color variations. Sap content at this grade is also very low. Select or ABC Grade ABC Grade wood flooring contains some knots, sap content, and color variations – more than AB Grade, but less than ABCD Grade. Natural or ABCD Grade ABCD Grade wood flooring, or Natural Grade, can also be referred to as mill run. At this grade, wood flooring planks have more noticeable and larger knots as compared to the previous two grades. They also contain more sap content and can offer more color variations. Rustic or DEF Grade These grades of wood provide a more rustic or traditional vintage look and feel. They tend to have a larger range in color variation and sap content. This type of wood will generally have more knots and fillers as well.