No structure is ever exactly level, plumb, straight, and true. Fortunately, such perfection is not necessary. However, problems develop when prefabricated components won't fit the spaces left for them or errors are so gross as to make the building unsightly, unusable, or unsafe. SOURCES OF TOLERANCE VALUES The following definition of tolerance as applied to building construction is widely accepted: the permitted variation from the lines, grades, dimensions, location, or alignment given in the contract documents. In the abscence of detailed specifications for cast-in-place concrete construction tolerances, ACI 117-90, "Standard Tolerances for Concrete Construction and Materials", is commonly accepted as a source of reasonable values for tolerances. But even when there is consensus on reasonable values for individual tolerances, problems can result from the following: the cumulative effect of several generally accepted individual tolerances; a lack of agreement on where and how the measurements are to be made; movement of the structure during and after construction; incompatibility of concrete tolerances with those for other trades and materials. EXAMPLES OF POTENTIAL PROBLEMS The article examines several areas where tolerance conflicts could cause problems. Examples are given concerning: reinforcement cover and member cross section; concrete joist construction; floor-to-ceiling clearances; concrete frame tolerances and interior finishes; the effect of post-tensioning on a building edge; and installation of windows and curtain walls.