In 1978, the U.S. Department of Agriculture Forest Products Laboratory in cooperation with the North American softwood lumber industry started the North American in-grade testing program. Over a period of 12 years, more than 70,000 pieces of full-size structural lumber in structural grades were pulled from hundreds of U.S. and Canadian mills. The lumber was tested to destruction to scientifically measure its performance characteristics. What evolved from the in-grade testing program are new design values for lumber in North America, based on the actual performance of full-size specimens. Four primary properties were evaluated: Fiber stress in bending, tension parallel to grain, compression parallel to grain, and modulus of elasticity. In 1991, the National Forest Products Association published a revision of the "National Design Specification for Wood Construction" (NDS). The NDS is a widely accepted set of design rules for wood structures, including certain kinds of formwork. As part of the revised NDS, new working stresses for wood, based on the in-grade testing, also were issued. These stresses are presented as base values that must be adjusted for size and for conditions of use to arrive at allowable stresses.