Just about every carpentry project will require a level. Walls need to be built vertically plumb, countertops, stairway steps and shelves must be exactly level, and roofs need to be pitched at a consistent inclination. I'd recommend you have at least two carpenters levels in your workshop; a 2 foot level to use for checking joists, studs and similar longer construction elements, as well as a 8 inch long torpedo level. These are easy to carry in a tool belt and handy for checking detail woodworking. A mason's level, at four feeet long or more, is useful for framing and stonework.

Care of Levels

You'll want to be careful with your levels, and not throw them them in with all the other tools in your toolbox. These are well tuned instruments and can break easily, so consider buying a protective carrying case along with level. If you are buying a used level, check it out on a level surface to be sure the marked fluid vials are still accurate. Just put one side of the level on an even, flat surface and read the gauge. Flip the level around 180 degrees and read the gauge again to see if the bubble shows the same reading. If it doesn't, most levels have calibrating screws in the mountings that can be adjusted so the readings are correct.

Bubble Gauges

A level will have at least one, and usually three, fluid-filled bubble gauges. These are sealed glass or plastic vials containing one air bubble showing the orientation of the level in three dimensional space. When the level is moved or tilted, the position of the bubble moves to reflect the new position of the level. Another name for this kind of level is a spirit level, as the gauge vials are traditionally filled with Ethanol (to prevent the fluid from freezing due to it's freezing point of −114°C).

The three gauges correspond to the three dimensions, horizontal or level, vertical orientation, or plumb, and angle, or pitch.

Digital Electronic Levels

The latest advance in these tools is the battery-powered digital readout level. The advantage here is the precision of the readout versus visual judgement of the bubble in the vial. Instead of gravity, these use laser positioning to calculate levelness. Using one is simple; point the laser at the surface you want to measure, and the laser flashes until the surface is moved to a level position.

Digital laser levels have come a long way from when the spirit level was invented by Melchisedech Thevenot, an amateur scientist, some time before February 2, 1661.