A master limited partnership (MLP) is organized as a limited partnership, which is usually a private investment arrangement. However, it trades daily on public stock markets. Investors in MLPs buy units (as shares are called) on a national securities exchange. Investors can see the prices of and sell their units at any time. (Selling units is much harder with private limited partnerships). MLPs were created to promote investment in some capital-intensive industries, presenting investors with a chance at higher returns and greater diversification than just a single stock. MLPs do offer some growth and chance for profit, but they're most famed for providing substantial income via regular payouts, called distributions. These distributions usually are made quarterly. The Tax Reform Act of 1986 defined how these companies can be structured and the Revenue Act of 1987 limits what types of companies can ultimately become MLPs. To be considered an MLP today, the venture must focus on acquisitions relating to either real estate, natural resources, or commodities. The majority of MLPs are in the oil and gas industries.