| The clamp screw attached with the upper plate of a theodolite is called upper plate clamp screw. When it is tightened, the inner spindle gets fixed with the outer spindle and, thus, the upper plate gets fixed in position.
The manipulation of the upper plate and lower plate clamp screws provide three conditions:
- When both the upper plate clamp screw and the lower plate clamp screw are tightened, the instrument gets fully fixed.
- When the upper plate clamp screw is tightened and the lower plate clamp screw is opened, the instrument rotates on its outer axis, There is no relative motion between the two plate and the readings in the horizontal vernier scales do not change.
- When the lower plate clamp screw is tightened, and the upper plate is opened, the instrument rotates on the inner axis with outer axis fixed. The readings in the horizontal vernier scales change.
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