Passeriformes or passerines, is a order which includes more than half of all bird species. A notable feature of passerines is the arrangement of their toes (three pointing forward and one back) which facilitates perching. Sometimes known as perching birds or, less accurately, as songbirds, the passerines form one of the most diverse terrestrial vertebrate orders, with over 5,000 identified species. It has roughly twice as many species as the largest of the mammal orders, the Rodentia. It contains more than 110 families, the second-most of any order of tetrapods (after Squamata, the scaled reptiles).