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The biological answer is that hardwoods produce seeds with some sort of covering, such as a fruit or a nut. Softwoods produce seeds with no covering—they fall to the ground. Hardwoods tend to be more dense, but the hardness or density of the wood is not the means of classification. (Balsa wood, the lightest of all woods, is considered a hardwood). But in the lumberyard, structural lumber is softwood because it is easy to work with and the least expensive. Hardwood is more expensive and is used mainly as a finishing material.