If a wall is not plumb and level from the outset, the wall will be unstable. Not only will the wall look imperfect but it is likely to have a shorter life span than a wall built correctly. If a building or a meeting of walls is not square, then those walls can also be unstable and the house an irregular shape.
Plumb means to be perfectly vertical, directed to the gravitational centre of the earth. If a wall is not vertical it can be very unsteady, making it weak. If walls for a house are not vertical there will be problems making it square and level. There will also be problems when it comes to roofing.
A plumb bobwas the original tool used to measure the vertical accuracy. A plumb bob is a weight, usually with a pointed tip attached to the end of a line, which, when steady, indicates the vertical line. After the invention of the spirit level in the industrial revolution the plumb bob was largely replaced.
Level means perfectly horizontal. It is important in wall building that the surface or foundation you are building on is perfectly level before you begin, or the wall itself will never be level.
Being square does not mean making the house square but means that the walls come together at a right angle (90°). To get a right angle, the Pythagoras theorem or the 3, 4, 5 rule is used.For more information, see our section: How to lay the first three courses?