These notes support the Painting and Decorating Course run by The DIY School in the North West in Stockport near Manchester.
A plumb line and bob will always give you a true vertical line when allowed to hand freely. Use is to create a guide line using a pencil. Avoid that line being directly under a paper join as the pencil graphite may bleed through onto the surface of the paper. Levels are good for the same purpose but a range of lengths would be required fit all tasks and a very long one required to get a true vertical line on a a starir well for example.
Laser levels, though expensive are good as it is not necessary to mark the wall and can be used over a projection such as a window sill where it can be difficult to get a perfect guide line with a plumb line or level.
When using a plumb line avoid it “sticking” to a sized wall or resting against the edge or something that could give a false reading.
Always test you level by turning the level round on the same surface. The level should read the same result whichever way round you uses it.