Plumb means perfectly vertical.

It comes from the Latin word for lead, because plumb bobs were originally made from lead.

When a plumb bob is hung from a string, gravity draws the weight downwards, and the string forms a vertical line.

Before people started using spirit levels, a plumb bob was the most reliable way of checking that a wall was vertical.

They are still sometimes used to line up a vertical point over a mark, such as a surveying instrument or staircase post.

Checking walls

The easiest way to check walls for plumb is to put a spirit level on the wall. If the bubble is not in the centre of the vial, you'll need to find out how far out of plumb the wall is by moving either the top or bottom of the level away from the wall until the bubble finds centre. Then measure the gap between the level and the wall.

Note that you only need to measure the gap at the height you're working to. That is, if the cabinet height is 870 mm above the floor, that will be the point where you should take the measurement.

When it comes to installing the cabinets, the gap can be wedged or packed, and a filler piece inserted between the cabinet and wall.

We'll talk more about this in the units Adjusting cabinets on-site and Installing cabinets on-site.

Using a level with a straight edge

Spirit levels come in lengths ranging from pocket sized mini levels through to builders' levels 1800 mm long or more. The longer the level is, the more accurate your measurements will be. However, a long level isn't much good if you can't fit it into the space you're working in.

Most kitchen and bathroom installers carry a short level for cramped spaces and a 1200 mm level for general use.

If they need to check a level over a long distance, they use a straight edge and sit the level on top.

You can buy a straight edge made from aluminium.

Alternatively, you can use a board that's perfectly straight, such as a kickboard or long strip of plywood.

Obviously, a straight edge needs to be perfectly straight to get a true reading, so if you're using a re-sawn board you should sight along its length to check that there's no hollows or high spots.