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The limit or set of limits placed on a measured product or process characteristic.

The Target Value (T) with a +/- tolerance around it is a common way of expressing specifications. The highest allowable value is called the Upper Specification Limit (USL) and the lowest allowable value is called the Lower Specification Limit (LSL).

In some cases only a maximum allowable value is specified (cycle times, flatness, etc.) and is expressed as the Upper Specification Limit (USL).


An engineer specifies the outer diameter of a tube as 0.250" +/- .003". The Target (T) is .250". The Upper Specification Limit (USL) is .253" and the Lower Specification Limit (LSL) is .247".

A company doing property reports has an agreement with their customers that it will not take longer than 24 hours to issue a report. In this case the Upper Specification Limit (USL) is 24 hours. The Lower Specification Limit (LSL) is 0 hours. There is no Target specified.