This part of IEC 60122 has been compiled in response to a generally expressed desire on the part of both users and manufacturers for a guide to the use of quartz crystal units for filters and oscillators so that the crystal units may be used to their best advantage. It draws attention to some of the more fundamental questions which should be considered by the user before it places its order for a unit for a new application, and in so doing will, it is hoped, help ensure against unsatisfactory performance, unfavourable cost and non-availability. It is not the function of this standard to explain theory, nor to attempt to cover all the eventualities that may arise in practical circumstances. Lastly, it should not be considered as a substitute for close liaison between manufacturer and user. Standard specifications, such as those of the IEC of which this guide forms a part, and national specifications or detail specifications issued by manufacturers, will define the available combinations of the resonant characteristics and the temperature characteristic. These specifications are compiled to include a wide range of quartz crystal units with standardized performances. It cannot be over-emphasized that the user should, wherever possible, select the quartz crystal units from these specifications, when available, even if it may lead to making small modifications to the circuit to enable the use of standard resonators. This applies particularly to the selection of the nominal frequency.