The police radio 10-code system represents one of law enforcement’s most enduring communication innovations. Developed in 1937 by Charles “Charlie” Hopper, the Communications Director for the Illinois State Police, these codes revolutionized how officers communicated over radio frequencies. During the early days of police radio technology, equipment was primitive by modern standards—transmitters were weak, receivers produced poor audio quality, and officers had to press a button and wait for tubes to warm up before speaking. These technical limitations made lengthy conversations impractical and often unintelligible.
Hopper recognized that reducing common phrases to brief numerical codes would solve multiple problems simultaneously. Short transmissions required less power, reducing strain on limited battery supplies. Brief codes were easier to understand through static and interference that plagued early radio systems. Standardized numbers eliminated confusion caused by regional accents, poor articulation, or background noise that made words difficult to distinguish. The “10” prefix served a practical purpose—it gave vacuum tubes in early radios time to warm up and stabilize before the important information (the second number) was transmitted.
The system spread rapidly throughout American law enforcement during the 1940s and 1950s as radio communication became standard in police work. Departments across the country adopted 10-codes, though variations developed as different agencies customized codes for their specific needs. By the 1960s and 1970s, 10-codes had become deeply embedded in police culture, immortalized in television shows, movies, and popular culture. The phrase “10-4” entered everyday American vocabulary as a synonym for acknowledgment and agreement, demonstrating how thoroughly these codes penetrated public consciousness.
Today, despite technological advances that have eliminated the technical limitations that originally necessitated 10-codes, these numerical signals remain central to police communication in many jurisdictions. They represent tradition, professional identity, and efficient communication that has served law enforcement well for over eight decades.
Common 10-Codes: Essential Police Radio Signals
10-4 – Message Received/Acknowledged
The most famous police code in American culture, 10-4 simply means “message received” or “I understand.” When an officer responds “10-4” to dispatch instructions or another officer’s transmission, they’re confirming they heard and comprehended the information. This code appears countless times during every shift as officers acknowledge assignments, confirm receipt of information, and indicate understanding of instructions. The universal recognition of 10-4 extends far beyond law enforcement—truckers, military personnel, and civilians use it colloquially to mean “okay” or “understood.” Its simplicity and clarity make it the perfect acknowledgment code, requiring just two syllables to confirm successful communication.
10-20 – Location/What is Your Position
10-20 asks for or provides location information, one of the most critical pieces of data in police work. When dispatch asks “What’s your 10-20?” they want to know the officer’s current location. Officers might respond “10-20 is Main Street and Fifth Avenue” or simply give their location if context is clear. This code appears constantly in radio traffic as dispatchers track unit positions, officers coordinate responses, and supervisors monitor coverage across their jurisdictions. Accurate location information ensures help can reach officers quickly during emergencies and allows efficient resource deployment. The 10-20 code has also entered popular culture—asking someone’s “twenty” means asking where they are, demonstrating how police terminology influences everyday language.
10-33 – Emergency Traffic/Officer Needs Help
10-33 represents one of the most serious codes in police radio communication. When an officer calls 10-33, they’re indicating an emergency situation requiring immediate assistance. This code instantly clears all non-emergency radio traffic, allowing the officer in distress to communicate freely. All available units respond immediately to provide backup and support. The 10-33 code might indicate an officer is under physical attack, facing an armed suspect, dealing with a violent crowd, or confronting any situation where their safety is compromised. Some jurisdictions use different codes for officer emergencies (like 10-99 or “officer needs assistance”), but 10-33 serves this critical function in many departments. The gravity of this code means officers use it only when genuinely necessary, knowing it will mobilize massive resources and create significant concern among fellow officers.
10-97 – Arrived at Scene
10-97 notifies dispatch and other units that an officer has reached their assigned location and is beginning to handle the call. This code serves multiple important functions in police operations. It allows dispatchers to update their tracking boards showing which units are on-scene versus en-route. It provides a timestamp for documentation and response time calculations. It alerts other responding units that someone is already addressing the situation. It also serves officer safety purposes—if an officer calls 10-97 and then goes silent for an extended period during a potentially dangerous call, dispatch knows to check on their welfare. Officers typically follow 10-97 with their unit number: “Unit 12, 10-97” tells everyone that Unit 12 has arrived and is now handling the call.
10-98 – Assignment Completed/Available for Service
When officers finish handling a call and complete all necessary documentation, they radio 10-98 to indicate they’re available for new assignments. This code tells dispatchers the officer has cleared from their previous location and can respond to pending calls. The 10-98 transmission closes out the call in dispatch records and updates resource availability throughout the jurisdiction. Officers might add information like “10-98, report filed” or “10-98, transport complete” to provide context about what actions they took. This code maintains the flow of police work, ensuring dispatch knows which units can handle new emergencies versus which are still tied up with previous assignments.
10-0 – Caution/Use Discretion
10-0 warns officers to proceed carefully or exercise particular discretion in their approach or actions. Dispatch might add this code to call assignments involving known dangerous locations, subjects with violent histories, or situations requiring tactical awareness. For example, “respond to disturbance at 123 Main Street, 10-0, history of weapons” alerts the officer that extra caution is warranted. Officers might also use 10-0 when communicating sensitive information over radio, signaling that discretion is needed in discussing details. This code enhances officer safety by ensuring appropriate awareness and preparation when approaching potentially hazardous situations.
Additional Essential 10-Codes
10-6 – Busy/Stand By: Indicates an officer is temporarily unavailable for new assignments while handling current tasks.
10-7 – Out of Service: Officer is unavailable for calls, typically used during meal breaks or when handling administrative duties.
10-8 – In Service/Available: Officer is back in service and available to handle calls after being 10-7 or otherwise unavailable.
10-15 – Prisoner in Custody: Officer has arrested someone and is transporting or booking them.
10-23 – Arrived at Scene (some jurisdictions use this instead of 10-97): Indicates officer has reached the call location.
10-76 – En Route: Officer is traveling to an assigned location, different from 10-97 which means they’ve arrived.
10-100 – Dead Body: Officers have discovered or are investigating a deceased person, triggering specific protocols for detectives and medical examiners.
The Shift to Plain Language Communication
Despite the long history and cultural significance of 10-codes, American law enforcement is experiencing a significant shift toward plain language communication. The movement gained momentum after the September 11, 2001 terrorist attacks, when the 9/11 Commission Report identified communication failures between different agencies as contributing to tragedy and confusion during the response. When police, fire, federal agents, and emergency management personnel from different jurisdictions attempted to coordinate, they discovered their 10-codes didn’t match—one agency’s 10-33 might mean something completely different to another agency.
The Department of Homeland Security and Federal Emergency Management Agency began strongly encouraging plain language for multi-agency incidents, particularly during disasters, terrorist attacks, and other major emergencies requiring coordinated response from multiple jurisdictions. Plain language means saying exactly what you mean: “Officer needs assistance” instead of a numeric code, “Arrived at scene” instead of 10-97, “What is your location?” instead of “What’s your 10-20?”
Proponents of plain language argue it eliminates confusion, requires no specialized training for new personnel or assisting agencies, and ensures that anyone monitoring radio traffic—including supervisors, allied agencies, and even civilian personnel during emergencies—understands exactly what’s happening. They point out that modern digital radio systems have eliminated the technical limitations that originally made codes necessary. Clear digital audio, stronger signals, and better equipment mean officers can communicate in complete sentences without the problems that plagued 1930s radio technology.
However, the transition faces significant resistance from officers and departments that view 10-codes as part of their professional identity and culture. Many officers argue that codes are actually faster than plain language, allowing more information transmission in less time, which matters during rapidly developing emergencies. They note that codes provide some operational security when members of the public monitor police scanners, preventing detailed tactical information from broadcasting to everyone listening. The brevity of codes also reduces radio congestion, keeping channels clear for other urgent traffic.
The result is a mixed landscape across American law enforcement. Some departments have completely eliminated 10-codes in favor of plain language. Others use plain language for major incidents but retain codes for routine operations. Many maintain traditional 10-codes for internal communications while switching to plain language when working with outside agencies. This compromise allows departments to preserve their culture and communication efficiency while maintaining interoperability when needed.
Federal agencies including the FBI, DEA, and ATF primarily use plain language, having made the transition years ago. Many large metropolitan departments have adopted plain language policies, though informal 10-code usage often persists among officers. Smaller departments and rural agencies tend to maintain 10-codes more consistently, viewing them as part of their traditions and effective communication practices.
The Cultural Significance of 10-Codes
Beyond their practical communication functions, 10-codes carry deep cultural meaning within law enforcement. Learning the codes represents a rite of passage for new officers—mastering this specialized language marks the transition from civilian to police professional. When rookie officers use codes naturally and appropriately, veteran officers recognize they’re becoming integrated into the profession.
The codes also create a sense of shared identity among officers from different departments and generations. A veteran officer who learned 10-codes in the 1980s shares this linguistic bond with today’s rookies, connecting decades of law enforcement experience through common terminology. This continuity of tradition strengthens the law enforcement community and maintains connections to policing’s history and evolution.
Popular culture has amplified 10-codes’ significance, with countless movies, television shows, and songs incorporating police radio communication. Shows like “Adam-12,” “Cops,” and “Live PD” familiarized the public with codes like 10-4, 10-20, and others, making police communication part of American cultural literacy. This exposure has made 10-codes recognizable even to citizens who’ve never worked in law enforcement, demonstrating their impact beyond professional circles.
Training and Code Proficiency
Police academies dedicate substantial training time to radio communication and 10-code usage. Recruits memorize common codes, practice using them in simulated scenarios, and demonstrate proficiency through examinations and practical exercises. This training emphasizes not just knowing what codes mean but understanding when and how to use them appropriately.
Field training extends this education as new officers apply classroom knowledge to real-world situations. Field training officers evaluate recruits’ radio communication skills, correcting improper code usage and reinforcing professional radio discipline. Officers learn that effective radio communication requires more than knowing codes—it demands clear articulation, appropriate brevity, proper procedure, and calm professionalism even during stressful emergencies.
Departments maintain radio communication standards through ongoing training, periodic evaluations, and supervisor monitoring. Dispatchers and supervisors who hear improper radio usage provide immediate correction, ensuring all officers maintain professional communication standards. This quality control prevents bad habits from developing and ensures consistent, clear communication across all shifts and units.
Conclusion:
Police radio 10-codes have served American law enforcement exceptionally well for over eighty years, transforming police communication from chaotic, inefficient exchanges into disciplined, professional systems that support officer safety and operational effectiveness. Whether departments maintain traditional codes, adopt plain language, or use some combination, the underlying principle remains constant: radio communication must be clear, concise, professional, and effective.
The codes represent more than mere abbreviations—they embody radio discipline that keeps channels clear for emergency traffic, ensures information flows efficiently, and maintains professional standards during all communications. When officers use 10-4 to acknowledge instructions, 10-20 to report locations, or 10-33 to summon emergency help, they’re participating in a communication tradition that has protected countless officers and served communities throughout America.
For aspiring law enforcement professionals, mastering 10-codes (or plain language alternatives) represents essential preparation for police careers. For researchers, journalists, and citizens interested in law enforcement, understanding these codes provides insight into how police coordinate their activities and maintain operational effectiveness. For the broader American culture, 10-codes have become linguistic touchstones that connect policing’s past with its present and future, demonstrating how professional innovation can achieve both practical success and cultural significance that endures across generations.