R.L- The vertical distance of a point above or below datum line is known as the reduced level (RL) of that point. The RL of a point may be positive or negative according to the point above or below the datum.

Line of collimation- It is an imaginary line passing through the intersection of the crosshair at the diaphragm and optical centre of the object glass and its continuation. It is also known as the line of sight.

Axis of the telescope- This axis is the imaginary line passing through the optical centre of the object glass and the optical centre of the eyepiece.

Axis of bubble tube- It is an imaginary line tangential to the longitudinal curve of the bubble tube at its middle point.