A toy dog is a very small dog kept as a pet, as compared to a very small working dog, although both pet and working dogs can be very small. When an all-breed association or kennel club divides dog breeds into groups for purposes of competition, the Toy Group contains most of the smallest dog breeds in the club's registry. There has been much discussion as to whether the designation signifies only a dog's size (weight, height, or both), or also refers to how a breed is used or its temperament. This has sometimes led to misunderstanding or bad feeling in the world of purebred dogs; there seems to be no consensus.
It seems safest to define toy dogs as being determined by the size of the dogs, and leave discussion of function and temperament to the various breed clubs. However, this can also be problematic, as there is no firm agreement on what height or weight makes a dog a toy, and the upper height limit of some toy dog breeds is over 12 inches at the withers, which places these dogs within the height range of dogs in some other groups as well.
The use of the word "toy" to describe small dogs that belong to a toy breed is redundant and also incorrect, suggesting that the breed comes in different sizes—there is no such thing, for example, as a "toy Chihuahua"; all Chihuahuas are toy dogs. (However, some breeds do come in different sizes, such as Poodles, which come in standard, miniature, and toy varieties).