A term used to describe the yield of grapes at harvest, usually expressed in hectolitres of grape must or wine per hectare or in kilograms of grapes per hectare. Overseas, tons/acre is also commonly used. This is also used as a wine-law specification, expressed as a 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. With regard to 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 treatment of the vineyard from the outset. In the middle of the 19th century, the density was still 20,000 vines per hectare on average, with yields not exceeding 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 wine-growing region, and also depends on any regional regulations).