The good news is that all types of spirit levels work using the same basic principle. The flat-based body contains a vial filled with liquid. Within this liquid is a bubble of air which moves between two lines to indicate whether your surface is level (horizontal) or plumb (vertical).

Most models include one vial for horizontal levelling and one for verticals; some even include a third vial for levelling 45° angles. Laser levels are used to level surfaces from a distance.