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The term gas-to-dust ratio (GDR) refers to the ratio of the mass of gas to that of dust, which may be used for objects such as a galaxy, or a star-forming region. Sometimes the (analogous) dust-to-gas ratio (DGR) is used for the purposes. These are examples of ratios that are determined when possible, and then used to estimate one or the other mass when only one is otherwise known. Dust-to-metal ratios (DMR) are investigated and used analogously.