As they frame doors and windows, or set beams and posts and even install trim, carpenters must be mindful of accuracy in both the horizontal and vertical directions. A vertical framing member, such as a post or stud, is "plumb" when it's perfectly straight, and when a horizontal member has no tilt, it is "level." The tools that carpenters and homeowners use to determine plumb and level all rely in some way on gravity.
The Plumb Bob
The word "plumb" comes from the Latin word "plumbum," which means lead. It came to acquire the meaning "exactly vertical" because one of the original tools for making this determination consisted of a string and a lead weight. This tool, called a plumb bob, is in common use even today, and some carpenters consider it the most reliable method for truing objects in the vertical direction. Modern plumb bobs are usually made of brass -- not lead -- and they have a conical shape that tapers to a sharp point at the lower end.
The Spirit Level
The level commonly used by modern carpenters consists of a sealed glass tube filled with just enough water to leave a bubble. The tube is affixed to a piece of wood or a metal straightedge in such a way that tilting the straightedge moves the bubble away from the center of the tube. Pre-modern carpenters used spirit levels; they were so named because they were filled with a spirit -- usually alcohol. A carpenter who has a level with two tubes mounted in perpendicular directions can use the tool to determine plumb as well as level, and many do. Spirit levels may contain a third vial indicating a 45-degree inclination.
The reason that a plumb bob is more reliable than a level for truing objects in the vertical direction that the plumb bob doesn't have to touch anything, and it relies solely on gravity. You simply hang the plumb bob from an overhead beam, wait for it to stop swinging, and ensure that the object you're truing is the same distance from the string from its top to its bottom. You can determine plumb by holding a level against the side of the post or stud, but warping in the wood or a worn-out vial on the level can throw off your measurements.
The spirit -- or water -- level is the traditional tool for ensuring that objects have to tilt in the horizontal direction, but it isn't the only one, and it has limitations. It must sit flush on a flat surface for the measurement to be meaningful, so you can't use it to level an object with a curved surface, such as a bathtub or an appliance with a curved top. For this reason, many carpenters pack a laser level; its dead-straight laser establishes a level line from which to take measurements. You can use a laser level that has a vertical beam as well as a horizontal to determine both level and plumb.