On The DIY School Plastering courses you will use levels to ensure that dry lining or dot and dab is vertical
Always take care of a level and check it before use
To check that a level is still accurate, place it upon a surface and look at the position of the bubble. Now turn the level round on the same surface. If the bubble returns to the same place than the level is still accurate. If the bubble returns to a different place than the level is no longer accurate.
A level should give the same result on a given surface irrespective of which way round it is placed
Whenever possible keep your level flat on the ground or propped in a corner when not in use. The biggest cause of levels becoming inaccurate is them being dropped
Never use your level to hit plasterboard straight when dry lining.