Examples of some frequently used police scanner codes are "10-4," "10-20," "Code 217" or "Code Blue." Police codes are a fast, efficient way for police officers to relay information to one another. A detailed list of police codes is available from PoliceScannerCodes.net.
Police scanner codes use the numbers 10 or 11 as a prefix. Scanner codes also use the word "code" itself, followed by an identifying number or color. Police 10-codes can vary among the different jurisdictions that use them as a way for police, ambulance and fire departments to communicate with one another, especially in emergency situations. Numbered codes, known as scanner penal codes, identify criminal acts that have taken place and enable officers to relay sensitive information in the presence of a victim. Three color codes also exist: "Code Blue," "Code Red" and "Code Purple."
Police 10-codes became popular by the early 1940s, according to the Federal Emergency Management Agency's Plain Language Guide. Because outside parties could easily monitor early two-way radio communication, police forces needed a way to keep their correspondence private and brief. Because the beginning of each transmission was often unintelligible due to static, officers started their messages with the number 10 as a way to fill the initial gap in correspondence.