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

Spirit level

A spirit level or bubble level is an instrument designed to indicate whether a surface ishorizontal or vertical . Different types of spirit levels may be used by carpenters, stonemasons, bricklayers, other building trades workers, surveyors, millwrights and other metalworkers, and in some...

 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 level

Dumpy level

A dumpy level, builder's auto level, leveling instrument, or automatic level is an optical instrument used to establish or check points in the same horizontal plane...

s.

As well as improving ease of use, a plumb sight may also reduce parallax

Parallax

Parallax is a displacement or difference in the apparent position of an object viewed along two different lines of sight, and is measured by the angle or semi-angle of inclination between those two lines. The term is derived from the Greek παράλλαξις , meaning "alteration"...

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.