PL tones, also known as PL codes or Continuous Tone Coded Squelch System (CTCSS), are sub-audible tones transmitted simultaneously with speech in two-way radio communications. They function as a filter, allowing specific conversations and radio traffic to be heard while eliminating unwanted traffic from other users on the same channel and frequency. Unlike common misconceptions, PL tones do not provide encryption or security advantages. For example, if you have 15 radios using one frequency but only want to hear conversations from specific radios, you can use pre-programmed PL tones and codes to filter out unwanted traffic. The history of PL codes is quite interesting; Motorola developed this technology in the early 1950s, patenting it as “Private Line” or PL. Other manufacturers adopted similar technology under names like “Channel Guard” (General), “Quiet Call” (Ritron), “Quiet Channel” (RCA), and “Call Guard” (E.F. Johnson), though “PL” became the generic term. The United States Energy Information Administration later coined the generic name “Continuous Tone Coded Squelch System (CTCSS).” Typically, these tones range from 67 to 254 hertz and are considered sub-audible. Originally, the mobile radio industry started with 38 sub-audible frequencies, which have since grown to 50. Discount Two-Way Radio repair and service department can easily add PL tones into any RCA or Motorola two-way radio. There is no generic standard tone number assignment or code letter for a particular tone, but a chart of commonly accepted 50 tones is available. In addition to these standard PL tones, “Color Dot” frequencies (154.570 MHz for blue, 154.600 MHz for green) are also accessible through business two-way radios, originally part of the Multi-Use Radio Service (MURS) bands. CTCSS tones are used to manage shared frequencies, preventing interference and ensuring that radio receivers only activate for signals intended for them, which is particularly useful in industries like security where specific protection details require constant monitoring without interruption from unrelated calls.