Lumber comes in two types: rough and dressed. Rough lumber has not yet been sanded and smoothed for use. The standard sizes for rough wood are also called the nominal dimensions. Surfaced wood, also called "dressed" wood, has been sanded and smoothed. Therefore, it can be dressed on one or both side. Dressed wood is slightly smaller than the rough lumber size from which it was processed; however, it's still referred to by the nominal wood size. Standard lumber thicknesses are measured in inches or quarters of an inch.
Boards refer to wood less than 2 inches thick. Dimension is the term used for wood between 2 and 5 inches thick. Timbers refer to wood that is at least 5 inches thick. Standard lumber lengths are in 1-foot lengths up to 6 feet long. After this point, the standard length changes to 2-foot increments, starting with 8 feet long.
Standard lumber thicknesses come in 1, 2, 4, 6 and 8 inches. In general, 1-inch and 2-inch thicknesses are used for boards while 4-inch thicknesses are used for posts and structural supports and 8-inch thicknesses are used in landscaping, such as for retaining walls. According to "How to Build Your Own Greenhouse" by Roger Marshall, "lengths increase in 2-foot increments starting at 6 feet." So basically, the standard 2 x 4 board is a 2 foot by 4 foot square of rough wood.
Quarter Designation System
According to "Making Cabinets & Built-Ins" by Sam Allen, "the thickness of a rough cut board can be designated by a system called the quarter designation; the nominal thickness is designated in quarters of an inch." Therefore, 1-inch-thick rough-cut boards are called 4 by 4s." In this system, a 6/4 is 1.5 inches thick.
Processing Dimension Changes
These standard thicknesses are for the rough wood sizes. Dressed wood is processed from rough wood, resulting in finished wood being thinner or narrower than the rough wood from which it was processed. According to "Audel Complete Building Construction" by Mark Richard Miller, Rex Miller, and Eugene Leger, "the familiar quoted size 2x4 refers to a rough or nominal dimension, but the actual size to which the lumber is dressed is 1½ inches x 3½ inches". Dressed thickness is ½ inch to ¾ inch narrower than the nominal dimension. The dressed width is 5/16 to ½ inch narrower than the rough dimension. The thickness lost to finishing depends on whether or not one side or both sides of the lumber is finished.