Spirit levels are available in many sizes and shapes, made of wood, aluminium or plastic, some have fixed vials, others are adjustable. All spirit levels have one or more vials for vertical and horizontal use, some have 45 degree vials. Inside the vial is a fluid with an air bubble, when the bubble is centered between the two indicator lines the surface is level. The fluid is alcohol based with a colorant such as fluorescein, typically yellow or green, added to increase the visibility of the bubble.

Longer levels 48" or 72" give a more accurate reading and are used in construction projects, if a board is bowed shorter levels will be resting on the curve as shown in the exaggerated sketch below.

Torpedo levels have narrow ends to fit into confined spaces and are handy for mounting cabinets.

To check a level for accuracy, place it upon a surface that the level indicates to be level, turn the level end for end, if it still indicates the surface is level it is accurate.

Bubbles always run uphill so raise the end of the level you want the bubble to run to.

photo by Glenn McKecknie

An engineer's spirit level, or machinist's level is generally used to level machinery.

This level offers four integeral slides to transfer mounting points from object to wall,
use to mount pictures, towel bars, shelf brackets.

Available from Rockler.com