Floors can be uneven or angled, and large pieces of furniture such as bookcases need to be level for safety, utilitarian and aesthetic purposes. Construction and repair specialists use wood shims to fine-tune the level of surfaces. Wood shims are usually made of cedar, and are designed as thin wedges that can easily be cut by snapping perpendicularly. Levels are essential in evaluating whether furniture is level, independent of the angle of the floor.
Using a bookcase as an example, place the level on a flat surface such as the top or a fixed shelf. Adjust one end of the bookcase vertically until the bubble is centered in the level's window. This will provide information about how the floor is uneven.
Insert the shim between the floor and the bottom of the bookcase on the side that needs to be raised. You may need to stack shims in order to make it level.
Check the level to make sure the bubble is centered, signifying a level surface.
Pull the exposed end of the shim straight up to cut the end flush with the side of the bookcase. If necessary, use your screwdriver to push any exposed ends under the bookcase.
Use the level to check other areas of the bookcase to make sure that all horizontal surfaces are level. Repeat inserting or adjusting the shim(s) as necessary.