A technique of high absolute accuracy for measuring low-loss symmetrical linear passive 4-terminal networks is described. The method is well suited for the v.h.f. and u.h.f. ranges. The unknown network is inserted in turn near a high- and a low-impedance point along a low-loss uniform line short-circuited at each end. From measurement of the two resonant frequencies (coincident, or nearly so, by adjustment of line lengths) and knowledge of the line dimensions, the characteristic impedance Z0xand electrical lengthlxof the equivalent uniform line are computed. Errors are discussed and means suggested for reducing them. The technique has been explored in the frequency range 300–2800 Mc/s, using precise 50Ω coaxial lines. Connectors, adaptors and small-range adjustable stubs, with reflections mostly less than 10%, have been measured. The r.m.s. scatter in the measured results is of the order of 0.1% inZ0xand 0.01 cm inlx. The absolute errors are estimated to be only a little greater. The method should apply over wider ranges of frequency and to a greater variety of unknowns, including waveguide two-ports.