The essential features of a top quality spirit level are the vials which should be shockproof and virtually unbreakable and the method of fixing the vials into the body so that they cannot work loose
The body should be tough, robust and stable and the working faces should be accurately ground
An accuracy tolerance of ± 1mm per metre or 0.057° is often quoted
This tolerance indicates that the work will not be more than a maximum of 1mm per metre out of true if it has been levelled with the bubble central between the two gauge lines
Expressed another way, if a 1 metre level is lifted by 1mm at one end then the included angle at the other end would be 0.057° against the horizontal
