A flight feather that lies along a single horizontal row on the rear margin of the anatomical tail. Only the central pair are attached (via ligaments) to the tail bones; the remaining rectrices are embedded into the rectricial bulbs, complex structures of fat and muscle that surround those bones. Rectrices are always paired, with a vast majority of species having six pairs. They are absent in grebes and some ratites, and greatly reduced in size in penguins. [ http://en.wikipedia.org/wiki/Flight_feather#Rectrices ]