Lep is correct regarding the origin of Q signals but fails to mention they were radiotelegraphy signals...certain codes were picked because they have a unique sound when sent as a whole word...SOS, for instance, dit dit dit dah dah dah dit dit dit, is sent as a single letter and has a unique sound, as does CQ (Calling any station), dah di dah dit dah dah di da....and the 10 signals are for brevity, but what happens when agencies in the same general area use different signals...example, the Florida 10 code for Officer Needs Help is 10-24, in the APCO ten codes, 10-24 means finished with last assignment (10-98 in Florida)...and for the NYPD its 10-13, which is a request for weather conditions in Florida.....with modern radio technology, signals, codes, and such are really unnecessary, and I am a firm believer in using clear spoken voice, despite having worked 28 years for an agency that has used signal and ten codes for its entire existance