Merinos vary considerably in size, conformation, and extent of skin folds, but the prevailing trend in breeding is to develop sheep of medium size, with fair mutton conformation and a minimum of skin folds. The colour of their faces and legs is white. Although they have a considerable growth of wool on their faces, it is seldom extensive enough to cause wool blindness. The fine wool fibres of Merino fleeces are beautifully crimped. Through selective mating and crossing with other breeds, the Merino has served extensively as foundation stock in the creation of many useful breeds and strains of sheep. See also livestock farming: sheep (livestock farming).