Unit of measurement to represent the relative density or specific gravity of grape must, which is the mass (weight) in relation to the volume. This mass, also called extract, consists of the dissolved substances in the grape must. This is mainly sugar (fructose, glucose), but also acids, minerals, phenolic compounds, proteins and others (these substances are then partly found in the total extract of the wine). The specific gravity of grape must is always greater than 1.0 (water), the difference is largely due to the sugar content. The difference between the "weight of a certain volume of must" and the "weight of the same volume of water" is called the weight ratio. The measurement is made by means of areometer (plummet), pycnometer and refractometer (light refraction).