Since World War II, radar systems engineers have used letter designations as a short notation for describing the frequency band of operation. This usage has continued throughout the years and is now an accepted practice of radar engineers. Radar-frequency letter designations are used for the following reasons: 1) They provide a convenient method for describing the band in which the radar operates without the need for awkwardly stating the limits of the frequency in numerical terms. For example, it is more convenient to say an L-band radar than a 1215 MHz to 1400 MHz radar. This is especially important in titles of published papers on radar, in advertising of radar systems and components, or in any other situation where a short notation is desired. 2) In military radar systems, the exact frequency of operation cannot usually be disclosed, but it is permissible in many cases to describe the band in which it operates. The letter designations permit this. 3) Each radar-frequency band has its own particular characteristics. Thus an X-band radar will be different from an S-band radar. The letter designations are often used in this manner to indicate the particular nature of the radar as it is influenced by its frequency. There are vast differences in characteristics, applications, and environmental constraints which distinguish radars in the different bands. It is the need to communicate concisely the whole set of characteristics which are shared by S-band radar, as distinguished from L-band radar, C-band radar, and the otherres, which requires the established usage of letter designations.