The level is a very basic symbol of Freemasonry, as a Mason must pledge to be "on the level" in all his dealings.
The two bottom limbs were set upon the object to be leveled. A weight on a string swung in the center notch, and the level was checked by reading the perpendicularity of the string. You can make a very accurate level by making a triangle or t shaped form of any size, hanging a plumb bob from the apex or top to swing in a cutout notch, and calibrating the string position by checking where it falls when the limbs are aligned with still water.
(Text provided by Todd Pierce, Mason)