The notarium, a group of fused thoracic vertebrae, is characteristic of birds of five orders and is found in one or more families of five more orders of non-passerine birds. Sixteen patterns of variation in the number of vertebrae in the notarium and of unfused vertebrae between it and the synsacrum were found. The occurrence of these patterns in the groups having a notarium is tabulated. Selective factors favoring the evolution of fusion of these vertebrae may have included the shock induced by landing on hard substrates or of striking prey and the prevention of downward bending of the ends of the thoracic portion of vertebral column while the birds are in flight. Phylogenetic implications of the presence of a notarium in several groups are discussed.