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Dielectric permittivity is the quantity that relates the electric displacement in a material to the applied field, written as a complex number whose real part describes stored energy and whose imaginary part the loss. Both parts depend on frequency and temperature, so a value quoted without them means little. At microwave frequencies the measurement is usually made by placing the specimen in a resonator or a transmission line and comparing the response with and without it; microwave dielectric ceramics are commonly characterised by two complementary methods, one based on the resonant frequency and quality factor, the other on reflection ^[1]. Thin sheets and liquids are more conveniently handled with an open-ended coaxial probe, whose fringing field senses only the near surface and which needs careful calibration to give absolute values ^[2]. High-permittivity materials are hard to measure in a closed cavity because the specimen perturbs it too strongly, and an extra-cavity arrangement avoids that ^[3]. Predicting the effective permittivity of a mixture from its constituents and their arrangement is a separate problem, addressed by mixing rules that take the inclusion shape into account ^[4]. Data-driven models fitted to measured spectra now serve the same purpose for heterogeneous materials, wood included ^[5].