K9 is a numeronym which stands for “dog,” usually in reference to domestic dogs. The term is most often used to describe working dogs, especially law enforcement and military dogs, in keeping with the military's famous love of shorthand, acronyms, and numeronyms. It may also be used in busy veterinary clinics and animal shelters to quickly denote “dog” on notice boards for the purpose of organizing surgeries and keeping track of the animals in the facility, as the animal's name may not always provide a clue as to what species it is.
Numeronyms are words based on numbers, with the number standing in for a particular sound. In the case of “K9,” when the letter and number are sounded out, the word reads “kay-nine,” a clear reference to “canine,” the term used to refer to members of the Canidae family, which includes dogs, foxes, and wolves. While numeronyms seem to be especially common in the era of texting, with people using terms like “l8r” for “later,” people have actually been creating numeronyms for a very long time, for the same reason that texters do: writing out a numeronym takes less time than writing out the full word.
In the case of police dogs, people use “K9” in labeling such as the notices on police cars which alert people to the fact that there is a dog inside, and on the uniforms worn by police dogs. K9 units can be found working all over the world, doing everything from walking a daily beat to looking for signs that someone is transporting drugs. The dogs and the handlers both receive extensive special training to ensure that they are suited for the job, and a good working police dog can be an extremely valuable commodity.
The military also utilizes “K9” in its reference to working dogs. Military dogs perform many of the same tasks that law enforcement dogs do, ranging from keeping an eye on public safety to searching for bombs. Military dogs are given training very similar to that used for police dogs, and they are also very valued commodities. Breeds such as German Shepherds, Labradors, Bloodhounds, and Rottweilers are all used routinely in police and military work.
When approaching or being approached by a K9 unit, people should remember that although working dogs may resemble pets, they are working animals, and they need to be treated with care. People should never pet or approach a working dog without permission from the handler, and they should be aware that many handlers prefer that their dogs not be distracted while at work. Military and police dogs are also treated legally like human police officers and members of the military, which means that an attack on a working dog will be treated extremely seriously, especially if the dog is seriously injured or killed.