This part of IEC 60444 describes two methods for determining the spurious (unwanted) modes of piezoelectric crystal resonators. It extends the capabilities and improves the reproducibility and accuracy compared to previous methods. The previous methods described in IEC 60283 (1968) were based on the use of a measuring bridge, which applies to non-traceable components such as variable resistors and a hybrid transformer, which are no longer commercially available. Method A (Full parameter determination) Full parameter determination allows the determination of the equivalent parameters of the spurious resonances and is based on the methods described in IEC 60444-5 using the same measurement equipment. It is the preferred method, which can be applied to the measurement of low and medium impedance spurious resonances up to several kΩ. Method B (Resistance determination) Resistance determination should be used for the determination of high impedance spurious resonances as specified, for example for certain filter crystals. It uses the same test equipment as method A in conjunction with a test fixture, which consists of commercially available microwave components such as a 180° hybrid coupler and a 10 dB attenuator, which are well-defined in a 50 Ω environment. This method is an improvement to the “reference method” of the obsolete IEC 60283.