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Accompanying the improvement of electrical insulating materials, reflected in higher and higher resistivities, is the need for greater confidence in bridge performance with respect to precision and accuracy. To establish such assurance, standards extending to the upper limit of the bridge are required. This paper describes the design and calibration of three-terminal wye (Y) network standards in the teraohm (1012 ohms), and multiteraohm (1014 ohms) ranges which qualify for this purpose. The Y configuration consists of a geometric arrangement of three resistive components in which one end of each component is connected to a common terminal, while the other ends of the branches constitute the independent terminals of the network. A particular feature is that no solid insulation is used within the assembly to provide mechanical and electrical isolation of the resistance elements. The burden of providing this isolation rests with the insulation of the terminal block of the bridge to which the standard is secured. A further desirable feature is that the resistance elements, none of which exceeds 109 ohms, can be calibrated individually without removing, exposing or in any way handling, or disturbing the Y structure. These standards, developed at the Bell Telephone Laboratories and manufactured and calibrated by the Leeds and Northrup Co., have been calibrated also by the National Bureau of Standards. Both organizations report results with a precision of at least one part in one thousand and with an uncertainty not exceeding three parts in one thousand for the 1012-ohm standard.