The sei whale is smaller in size than the fin whale (Balaenoptera physalus), and can be distinguished from this similar species because it has symmetrical colouring on the lower parts of its head. It is also similar to Bryde's whale (Balaenoptera edeni), but has only one ridge on the upper surface of the head, whereas Bryde's whale has three. The 'blow' or spout of the sei whale is a single thin cloud, which reaches about three metres in height. The skin is a mottled dark grey colour, with white grooves along the paler underparts. The baleen is grey to black with paler fringes and less than 80 centimetres in length. The dorsal fin is obvious, has a slightly hooked shape and is located two-thirds along the length of the body. The common name 'sei' arose from the arrival of this whale off the coast of Norway tending to coincide with that of coalfish 'seje'.