A mixed-breed dog, also called a mutt, mongrel, tyke, cur, bitzer, or random-bred dog, is a dog that has characteristics of two or more types of breeds, or is a descendant of feral or pariah dog populations. The term mixed-breed can be a misnomer. Along with the term purebred dog, the idea that such dogs are a mix of defined breeds stems from an inverted understanding of the origins of dog breeds. Pure breeds have been, for the most part, artificially created by human selective breeding to enhance one or another desired physical or temperamental characteristic. Dogs that are not pure bred are not necessarily a mix of such defined breeds. In contrast, the term crossbreed refers to dogs produced by the intentional crossing of two different known purebred dog breeds. The term "mutt" or mixed-breed generally refers to a dog of unknown descent. Dogs interbreed freely, except where extreme variations in size exist, so mixed-breed dogs vary in size, shape, and color, making them difficult to classify physically.