Mills usually categorize framing lumber generically as studs, two-by-fours, two-by-sixes or four-by-fours, among other terms. The grading system for lumber indicates whether it is appropriate for framing.
Softwood is the prevalent framing species, with grading systems based on structural requirements. Hardwoods such as oak, ash and mahogany also have grades, but for appearance only.
All the information you need to know about framing lumber should be printed on the individual boards or the bulk unit of boards. The information, also known as the grade stamp, includes the species, quality or number of defects, moisture content and origination.
Common or standard-grade lumber is the most affordable. It may be warped, twisted or contain small defects. If you purchase common lumber in bulk, most lumber yards allow you to reject pieces that you can't use or don't want. Common-grade lumber, also called utility- or construction-grade, should be clearly stamped "common" or "standard" or an abbreviation of one of those terms.
Select grade is somewhat rare and at the other end of the scale from common or standard grade, and it's expensive. Use select grade if you're concerned about appearance. Select grade typically has four levels. Select or prime is knot-free, without warps, twists or defects. Select grade numbers 1 through 3 have increasing numbers of allowable defects but still fewer defects than common grade.
Framing lumber usually has been dried to a specific moisture level. Look for the "KD" or "S-Dry" stamp, meaning that it's kiln dried to a specific moisture level. "S-Grn" means the lumber is green and not safe to use until it dries. Don't use lumber over the 19 percent moisture level. Wet lumber warps and twists. Most lumber is fine to use, because dealers typically don't sell it wet, but occasionally truckers expose lumber to rain and deliver it wet. You can check lumber with a hand-held moisture meter if you're curious. If you're unsure about the moisture level or any other issue, ask for assistance.
The most common species used for framing lumber are Douglas fir, spruce, Southern pine and hemlock. These species differ somewhat in structural capabilities but all exceed regulations for structural strength and are fine for most framing applications. Because the lumber is often combined generically, the grade stamps may group two or more types together, such as "hem-fir" or "spruce-pine-fir."
Nominal Versus Actual
The majority of framing lumber is sized dimensionally. The two-by-four term is a nominal measurement, old-school reference to a 2-inch-thick, 4-inch-wide piece of rough lumber, which may still be available at some lumber yards or sawmills. The actual dimensions of a common two-by-four today are 1 1/2 inches by 3 1/2 inches. Dimensional lumber has been run through machinery for consistency. It's typically graded for moisture content and quality and is always the best choice for framing material.