FAQ Knowledge Articles

Plumb. Level. Square. True. These are alignment requirements of rectangular windows. Preached by installation guidelines throughout the industry, all four need to be satisfied for a proper installation. This is for good reason because these have so much to do with the appearance, and especially performance, of an installed product. Guesswork is not optional here.
While all four are quite specific, they are relative to, and affect each other. That’s helpful for the installer if he understands this relationship.

First of all, by quick definition:
  • Plumb means straight up and down, perfectly vertical. If a level (the instrument) is held against the vertical side jamb, you are measuring for plumb, not for level (a common misunderstanding).
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  • Level means not sloped at all, perfectly horizontal. The surface of a calm lake for example is not only flat, but also level. This would apply to checking the sill or head jamb for level.
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  • Square describes an object (such as a rectangular frame – opposing legs are equal length whereby side jambs are equal, and sill and head jamb are equal length) that has uniform right angles at each corner. This can be most easily verified by measuring diagonally corner-to-corner. Lengths should be equal.
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  • True means straight line in reference to the “legs” of the unit. Double hung side jambs are commonly not true because they are bellied out at the checkrail. This is probably the most commonly overlooked of the four parameters. This can be checked with a straight edge, or in the case of a very wide or tall multi-unit, a string line.
Here’s the relationship (assuming all legs are true)
  • Because plumb is at a right angle (90 degrees) to level, square is automatically attained if your window is level AND plumb.
  • If any one inside corner is 90 degrees, then so are the other 3 corners and frame is square
  • By the same token, if your unit is square and you either level the sill, OR plumb the side jamb, you get plumb, level and square.
  • If any leg is not true, excessively bowed for instance, this reduces the effective length of the leg and squareness of the unit is impossible.
When 2 equals 3 (again assuming all legs are true):
  • Level sill + plumb sides = square unit
  • Level sill + square unit = plumb sides
  • Plumb sides + square unit = level sill
Check all three as a cross check. Understanding these relationships will make installations a bit easier knowing how one parameter affects the other.