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These are some of the different types of levels you can use to establish true vertical and true horizontal.
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In making or repairing small rectangular objects, a square is invaluable in helping ensure the joints are tight and the overall shape is true and square. But for a fixed structure, the scale of a mere square, even a framing square, doesn’t permit it to provide all the answers.

Enter the level and its partner, the plumb. These tools are used to establish true vertical and true horizontal.

The key element in a level is the sealed glass or plastic tube containing water, alcohol, chloroform, or some other clear liquid. The tube or vial is slightly curved and has two parallel lines drawn at its center. The vial is nearly filled, leaving an all-important air bubble. The vial is then precisely mounted in the body of the level.

No matter what size the level, its function depends upon that tiny air pocket in the vial. Since the specific gravity of the fluid is greater than that of air, the bubble always rises to the highest point in the vial. When the frame of the tool is precisely level, the bubble will be aligned between the two hairlines at the center of the vial.

The bubble tube or vial can be mounted in a variety of instruments. Some are fixed permanently in place at the time of manufacture, others are adjustable or replaceable. the most common kinds of levels are described below:

Carpenter’s Level
These levels come in many sizes, as two-, four-, six-, and eight-foot models are commonly available. For the tasks most of us address, one each of the two-foot and four-foot varieties will be sufficient.

Two-foot levels consisting of a wooden body and one or more bubble tubes are generally called carpenter’s level. Typically, such levels are about three inches high and an inch or so deep.

Two-foot and four-foot models alike usually have three bubble vials, one at each end mounted transversely for establishing true vertical, and one mounted at the center along the length of level for horizontal leveling. For generations, carpenter’s levels have been made of handsome (and very stable) woods like rosewood, ebony, and mahogany.

Mason’s Level
Mason’s levels are usually four feet or longer. Two things to keep in mind: the longer the level, the greater the accuracy — and that when working in cramped quarters, an overlong level is useless. So having a nine-inch torpedo level in your kit is probably a good idea. There are smaller one, too, as short as an inch in length.

For odd jobs around the house, the two-foot length is easily stored, and used. The four-foot level is handiest for cabinet installation.

Vials are replaceable in many new models. Some levels also have a vial set at a 45-degree angle to the length of the tool. This allows you to determine proper position for braces and other angled pieces.

Torpedo Level
Typically nine inches long and tapered at the ends, the torpedo level is sometimes also known as a canoe or boat-shaped level. The body of the level contains two or three spirit tubes. The torpedo level is handiest when working in tight quarters. It’s small enough to be put to use in spaces that are too cramped for longer levels. The torpedo level will also fit comfortably into a pants pocket.

Line Level
Not much larger than the vial it contains, the line level is designed to be hung from a taut string stretched between two points. Hooks at either end of the line level’s body attach to the line at roughly the midpoint of the reach. As with other levels, when the bubble is centered between the vertical markings on the vial, the line to which the level is attached is true.

Line levels are used by masons, but carpenters, too, often find them handy when framing a new floor or ceiling, or in squaring off an old ceiling. Lining up concrete piers or fence posts, or checking the pitch of a driveway or gutter are other tasks that can be done conveniently with the line level.

Note that a sagging string will almost guarantee a misleading reading, so be sure to keep the string taut. Despite this precaution, however, you should keep in mind that even the tightest of strings sags perceptibly, which means a line level has limited accuracy. The line level should not be used where precision is all-important, but in foundation work and rough carpentry, it’s a handy tool indeed.

Water Level
The water level consists of a length of flexible plastic tubing or hose (typically, three-eighth-inch outside diameter, a quarter inch inside). At the ends of the tubing are rigid plastic cylinders set on bases that hold the cylinders upright. Water is contained within the device, usually with a few drops of food coloring added to make the water levels easier to read. On commercially available models, calibrations are printed on the cylinders.

Rather than relying on a single bubble, the water level relies on Pascal’s law which, in its simplest terms, states that water always seeks its own level. In practice, then, you set the two cylinders atop the two surfaces you want leveled with one another; the connecting tube can assume any position as long as it is below the level of the water-filled cylinders. If the surfaces are the same height, the water level in the two cylinders will be level; when the cylinders are not level with one another, the water in the device will be in the high zone at one end and the low zone at the other.

The water level is often used by foundation contractors, but also by carpenters, landscapers, plumbers, and other tradesmen. Locating a dropped ceiling, for example, is made simpler by sing a water level. Lining up footings for a deck (or the decking itself) is another task often tackled more easily using a water level than with a carpenter’s level or even a line level.

One key advantage of the tool is that is can be of virtually any size, giving its user the ability to level objects that are many feet apart. The length of the hose between the cylinders can be only a few feet (in leveling a pool table, for example) or a hundred or more feet in leveling the foundation of a building. The water level is effective with objects that are separated by some obstacle, like a tree or structure.