The sensitivity is an important specification for a spirit level; its accuracy depends on its sensitivity. The sensitivity of a level is given as the change of angle or gradient required to move the bubble by unit distance. If the bubble housing has graduated divisions then the sensitivity is the angle or gradient change that moves the bubble by one of these divisions. 2 mm (0.079 in) is the usual spacing for graduations; on a surveyor's level the bubble will move 2 mm when the vial is tilted about 0.005 degree.