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All About Levels, Plumb Bobs and Guide Lines

Levels are essential to many DIY jobs because they ensure that guide lines or marks are precisely horizontal (level) or vertical (plumb). You can also use them to set accurate angled guides.

Excerpted from Do It Yourself Home Improvement

Checking a Level's Accuracy

It is wise to check the accuracy of a level from time to time. Hold the level against a wall and draw a line that it indicates is horizontal. Turn the level around 180 degrees and draw a second horizontal line parallel to the first. Measure the gap between the two lines at several points. If the gap is uneven, the level is no longer accurate and should be discarded. Use the same principle to test the vertical accuracy of the level by measuring the gap between two lines that the level considers vertical.