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Plumb Site is a proprietary improvement, and a trademark under that spelling, for a spirit level with a viewing mirror to make it easier to use when setting verticals.
A plumb sight is a mirror inside a spirit level that allows the user to view the plumb vial from the front, as well as from the traditional side view, when performing a vertical leveling operation. The user no longer has to squeeze against a wall to see the bubble from the side, but can stand in front of the level and see its reflected image.
Although the use of this sight may be novel for levels, the idea of using a mirror to sight a spirit level bubble from a more convenient position has long been used with surveyor's telescopic dumpy levels.
As well as improving ease of use, a plumb sight may also reduce parallax errors. By using a sight that always views the bubble from the ideal location, risk of reading errors is reduced.
Plumb Site was invented in 1997, and is a feature across Kapro's range of spirit levels.