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IEEE Standard C37.118.1 defines requirements for the measurement and determination of a Phasor Measurement Unit (PMU). The PMUs must be tested to ensure that all PMUs on the power grid behave the same. In order to meet the requirement of PMU within specified uncertainty, the PMU calibration systems or calibrators must be traceable to known reference standards with an unbroken chain of calibration. If all the testing laboratories are traceable to reference standards, the test and equipment are similar, and staffs are qualified, then PMUs tested by all of them will have the same behavior. Inaccurate PMU calibration can lead to selecting PMUs that do not behave the same, which can have serious financial consequences to electric power utilities as well as reliability and safety consequences. The measurement of voltage and current phasors relative to absolute phase angles, i.e., relative to a nominal phasor synchronized to Coordinated Universal Time, UTC, is not a common measurement in most testing laboratories. This paper describes the basic concepts of phasor traceability and how they are applied to the PMU calibration system used at the National Institute of Standards and Technology (NIST). The reliable and accurate PMU calibration system is able to support PMU manufacturers and other testing laboratories to improve their products or calibrations to meet the IEEE Standard C37.118.1. Examples of improving PMU performance after feedback of NIST calibrations to the manufacturers will be described.