The majority of two-by-fours are for structural purposes, but they're also used for furniture, aesthetics and, yes, even aroma. Sometimes referred to as studs, they vary in weight, density, color, strength and resistance. Don't limit yourself to a single type when there are many to chose from.
Two-by-four is a figure of speech. Standard two-by-fours do not match their stated dimensions of 2 by 4 inches. The variance is because of milling needed to produce clean, smooth lumber. Before planing, two-by-fours have inconsistent dimensions of width and thickness, typically varying by about 1/4 to 1/2 inch along the length. This type of two-by-four is known as a rough two-by-four. There is limited availability of this type of stud, and it's cheaper. You may need to go to a lumberyard to find it. If you have planing equipment, you can plane it down yourself and save money or use it as-is. Dimensional studs are far more common; they're smooth with rounded corners after planing and consistently measure 1 1/2 inches in thickness and 3 1/2 inches in width.
One of the most commonly used two-by-fours for framing is made from softwood known as Douglas fir, with hemlock a close competitor. The two species share similar qualities of strength, appearance and durability and are often marketed together and sold as Hem-fir. They are sometimes marketed separately when graded for appearance, but the two species are interchangeable for most structural applications. Yellow or Southern pine, another softwood with a similar appearance, should not be confused with fir or hemlock; it's harder and more expensive. Other pine varieties include Ponderosa and Lodgepole, which are softer, less expensive and not as strong as fir, hemlock or Yellow pine.
Redwood two-by-fours are among the most expensive. The natural weather and insect resistance, deep red color and uniform quality make redwood studs ideal for outdoor furniture, decking, railing and applications involving hot tubs and saunas. Redwood is regulated by environmental laws that prevent large quantities from being harvested. Check with local suppliers for availability before planning to use redwood.
Almost nothing compares to the scent and decay resistance offered by cedar two-by-fours. With stability and consistently straight pieces, cedar is often used in homes with exposed beams, such as in cabins or lodges. The beauty and aromatic scent adds a natural, outdoorsy ambiance to any room.
Serving as an affordable alternative to cedar, spruce has excellent decay resistance with similar beauty. It has the highest strength-to-weight ratio of all the available types of two-by-fours. It's readily available at most home supply stores or lumberyards and is one of the straightest two-by-fours available. Use it indoors as you would any other stud or outdoors as a cheaper version of redwood.