Accurate measurements are arguably the most important part of carpentry. The old saying "measure twice, cut once" is as true today as it ever has been. Devices for measuring are plentiful, but there are a few that are mainstays in woodworking. With these measuring tools in your shop, you are ready for most carpentry jobs.
Metal Tape Measure
Metal tape measures are the workhorses of woodworking. They remain wound inside a hard case until you pull them out, and they retract automatically when you are finished. Most have a locking mechanism that prevents the tape from retracting until you are finished, and some use applied friction to steady the tape without needing a lock. A metal tab or lip on the end allows you to measure a long board with little or no assistance because the tab catches the end of the board, freeing you to pull it out to the length you need. Metal measuring tapes need care because if they become bent, they are ruined. They are not the best choice for measurements under one inch.
Rulers and Yardsticks
Although plain basic tools, rulers and yardsticks are important because they don't stretch and they don't bend or break easily. Picking up a ruler for a small quick measurement is faster than pulling out a tape measure. It also gives a more accurate measurement under the one-inch mark where many tapes are either blank or the area is covered by the metal tab. Yardsticks are not only functional in the same way rulers are, they also give you a clean edge for creating straight lines with a pencil.
Folding rulers have been around a long time. They are basically six-foot-long rulers that not only become portable by folding them down, they also open to just the right length and help measure accurately around angles.
Calipers are another tried and true tool used in carpentry. They are used to measure the thickness of an object to ensure a good fit. Early calipers did not rely on inch measurements. Instead they used trammel points, which are the tips of both sides of the measuring device in joinery work as shown in Colonial Williamsburg's descriptions of authentic Colonial tools.
A carpenter's square, also called a framing square, is not a square at all. It is actually shaped like an L. This tool allows you to ensure that an angle is perfectly square at a 90-degree angle, but that is only the beginning. For such a basic-looking tool, the carpenter's square allows you to simply calculate nearly any angle you want. The numbering down both legs of the square looks confusing, but once you understand how to use the numbers, they are valuable tools, according to Tim Carter of Ask the Builder.