Specific gravity compares the density of a substance with that of a reference material. For liquids and solids the reference is water at its densest, 4 °C, with the sample usually measured at 20 °C. It is synonymous with relative density, is dimensionless, and is abbreviated to sp. gr. In food plants it is a rapid proxy for concentration in syrups, brines, and fermenting worts, measured with hydrometers or digital density meters. Brewers, distillers, and syrup makers rely on it daily to track fermentation progress and standardize products.