Keiretsu refers to the horizontally and vertically linked industrial structure of post-war Japan. The horizontally linked groups include a broad range of industries linked via banks and general trading firms. There are eight major industrial groups, sometimes referred to as "Kigyo Shudan": Mitsubishi, Mitsui, Sumitomo, Fuyo, DKB, Sanwa, Tokai, and IBJ. The vertically linked groups (such as Toyota, Matshushita, and Sony) are centered around parent companies, with subsidiaries frequently serving as suppliers, distributors, and retail outlets. Common characteristics among the groups include crossholding of company shares, intra-group financing, joint investment, mutual appointment of officers, and other joint business activities. The keiretsu system emphasizes mutual cooperation and protects affiliates from mergers and acquisitions. Ties within groups became looser after the oil shocks of the 1970s as a result of decreasing dependence on banks for capital.