The National Design Specification for Wood Construction of the American Forest and Paper Association (2012 edition) provides the basic standards and specifications for sawn lumber and engineered wood (e.g., glued laminated timber [GLULAM]) in the United States. The second part of the National Design Specification (NDS), referred to as the NDS supplement, contains numerical values for the strength of different varieties of wood grouped according to the species of trees. Pieces of wood sawn from the same species or even the same source show a great variation in engineering properties. Accordingly, the lumber is graded to establish strength values. Pieces of lumber having similar mechanical properties are placed in the same class known as the grade of wood. Most lumber is visually graded. However, a small percentage is mechanically graded. In each grade, the relative size of wood section and the suitability of that size for a structural application are used as additional guides to establish the strength.