This part of IEC 1178 applies to quartz crystal units manufactured as custom-built products or as standard catalogue items and whose quality is assessed on the basis of capability approval. It prescribes the preferred ratings and characteristics, with the appropriate tests and measuring methods contained in the generic specification IEC 1178-1 and gives general performance requirements to be used in detail specifications for quartz crystal units. The concept of preferred values is directly applicable to standard catalogue items but does not necessarily apply to custom-built products.