An alcoholometer is a hydrometer which is used for determining the alcoholic strength of liquids. It is also known as a proof and Tralles hydrometer (named after Johann Georg Tralles, but commonly misspelled as traille and tralle). It only measures the density of the fluid. Certain assumptions are made to estimate the amount of alcohol present in the fluid. Alcoholometers have scales marked with volume percents of "potential alcohol", based on a pre-calculated specific gravity. A higher "potential alcohol" reading on this scale is caused by a greater specific gravity, assumed to be caused by the introduction of dissolved sugars. A reading is taken before and after fermentation and approximate alcohol content is determined by subtracting the post fermentation reading from the pre-fermentation reading.[5]