An often overlooked market structure is that of Oligopoly. This is far less competitive (and efficient) than the previous two. This is where a small number of large firms supply similar products in an industry. Each firms actions is influenced by the potential (forecast) retaliatory actions of its rival – which usually results in price stability (because noone wants to start a costly price war with its rivals!)
This one has quite an unusual SR equilibrium graph (the ‘Kinked’ Demand curve)…
Heres the PDF PIIGSTY Econ 101 #15 Oligopoly