In the classification of domestic birds a _variety_ is properly a color-variety of a breed. Thus in the Plymouth Rock breed there are six color-varieties--barred, white, buff, partridge, silver-penciled, and ermine (called Columbian); and in Fantail Pigeons there are six color-varieties--white, blue, black, red, yellow, and silver. Birds of the same breed (shape) and the same variety (color) may differ in some other character, as the form of the comb or the presence or absence of feathers in certain places. In accordance with such differences varieties are divided into _subvarieties_. Thus, in Leghorn Fowls the brown, white, and buff varieties have single-combed and rose-combed subvarieties. In any breed, variety, or subvariety certain families are sometimes distinguished for general or special excellence of form or color. Such a family is called a _strain_. =Systematic mixtures of breeds and varieties.= Although so many distinct varieties have been developed from common domestic stocks, the improved races do not always displace the mongrels. When the old mongrels disappear their place is often taken by a new mongrel stock produced by mixtures of the distinct breeds with each other and with the old mongrel race. The greater part of such stock is so mixed that its relation to any established breed could not be determined or expressed, but systematic mixtures are sometimes made, and to describe these the following terms are used: _Crossbred_--having parents of different, distinct breeds, varieties, or subvarieties. A Leghorn male mated with a Cochin female produces offspring each of which is in blood one half Leghorn and one half Cochin. _Grade_--having more than half of the blood of a breed. If the offspring of a cross such as is described in the preceding paragraph are mated with birds of one of the parent breeds, the offspring of this mating will have three fourths of the blood of that breed. If these in turn are mated to birds of the same pure breed, the offspring will have seven eighths of the blood of that breed. Animals bred in this way are called _grades_ until the process has been carried so far that they are practically pure-bred. Mongrel stock is often graded up in this way. As a rule stock that is seven eighths pure is not distinguishable from average pure stock of the same breed. =Pure-bred, thoroughbred, and standard-bred.= A _pure-bred_ animal is, strictly speaking, one having the blood only of the variety to which it belongs. From what has been said of the making of breeds and varieties it is plain that absolute purity of blood is not a universal attribute of well-bred domestic birds. A _thoroughbred_ animal is one that is thoroughly bred for some purpose or to some type. A _standard-bred_ animal is one that is bred especially to conform to requirements agreed upon by breeders and exhibitors.