Tolerances may be thought of as a recognition of the fact that a structure cannot be built exactly to nominal dimensions given in the plans and specifications. No structure is ever exactly level, plumb, straight and true. Fortunately, such perfection is not necessary. Wide dimensional variations may occur in practice and pass unnoticed by constructors, designers, and owners alike. Inherent in traditional on-site construction practices are many design and workmanship skills that conceal and correct for dimensional variations as they occur. Craftsmen have customarily fitted their own work into a building as they find it. However, problems and hardships develop when prefabricated components won't fit the spaces left for them or errors are so gross as to make the building unsafe or unsightly. Then the question must be answered: How far can the structure vary from planned line and dimension and still be considered to accord sufficiently with the plans? How much variation is consistent with full performance of the contract? This is a matter of tolerance, whether stated or implied. A good approach which has some merit, is to set up several classes of tolerances, something like the following: ordinary tolerances to provide the structure with basic service ability. Special tolerances to provide the structure with consistently close fit for auxiliary structural steel or precast or precast floor and wall units. Appearance tolerances to set a standard for exterior columns, beams, and walls where the applied finish cannot mask errors and unique high-precision requirements- such as for tool or turbine installations. One of the basic hazards in specifying tolerances is the possibility of more than one interpretation. Working of the specification is as important as the numerical values that are give. A good tolerance specification should include definitions of special terms and state the tolerances as nearly as possible in the same order as the construction sequence. It is also important to provide that in the event of conflict a variation permitted in one section of the specification for concrete work is not to be construed as permitting violation of more stringent requirements in another section.