Distinctive features of underground surveying are that stations are usually in the roof instead of the floor; the object to be sighted and the crosshairs of the telescope must be illuminated; distances are usually measured on the slope; either the transit tripod has adjustable legs or a trivet is used; and often an auxiliary telescope is attached to the transit, either at one end of the horizontal axis or above the main telescope, with the line of sight of the auxiliary telescope parallel to that of the main telescope. Horizontal and vertical distances are computed from slope distances and vertical angles. The transit is set up at one station, being centered by plumb, and the vertical distance from the station to the horizontal axis of the transit is measured. A plumb bob is hung at the next station, with a point on the plumbline marked by some form of clamping target. The vertical angle to the point so marked is measured, and the distance from horizontal axis to the target is taped.