The mass difference between equivalent volumes of must and water is almost entirely due to the dissolved sugar in the must. Since the alcohol in wine is produced by fermentation of the sugar, the Oechsle scale is used to predict the maximal possible alcohol content of the finished wine. This measure is commonly used to select when to harvest grapes. In the vineyard, the must density is usually measured by using a refractometer by crushing a few grapes between the fingers and letting the must drip onto the glass of the refractometer. In countries using the Oechsle scale, the refractometer will be calibrated in Oechsle degrees, but is an indirect measure, since the refractometer actually measures the refractive index of the grape must.
The Oechsle scale forms the basis of most of the German wine classification. In the highest quality category, Prädikatswein (formerly known as Qualitätswein mit Prädikat, QmP), the wine is assigned a Prädikat based on the Oechsle reading of the must. The regulations set out minimum Oechsle readings for each Prädikat, which depend on wine-growing regions and grape variety:
The sugar content indicated by the Oechsle scale only refers to the unfermented grape must, never to the finished wine.
The Baumé scale is occasionally used in France and by U.S. brewers, and in the New World the Brix scale is used to describe the readings of a refractometer when measuring the sugar content of a given sample. All of these methods are similar and the differences are more cultural than significant, but all are equally valid ways to measure the density of grape must and other sugar based liquids.