Designation for the yield of grapes during the harvest, usually expressed in hectolitres of grape must or wine per hectare or in kilograms of grapes per hectare. Overseas, tons/acre is also common. This is also used as a wine law specification, expressed as maximum yield in hectolitres per hectare, for certain wine quality levels and can vary greatly depending on the country, wine-growing region, individual sites or even certain wines. In terms of vine density, there has been an extreme reduction over the last two millennia. The Romans still recommended 50,000 vines per hectare; this ruled out mechanical cultivation of the vineyard from the outset. In the mid-19th century, the average density was still 20,000 vines per hectare, with yields of no more than 40 hectolitres per hectare. Today, the vines are planted at a distance of 1.5 to 2 metres from each other (this varies from country to country and from wine-growing region to wine-growing region, and also depends on any regional regulations).