At harvest time, AOC regulations stipulate grapes for Grand Cru vineyard must be picked with a potential alcohol level of at least 11 percent, at least 10.5 percent for Premiers Crus and 9.5 percent for AOC Chablis vineyards. Yields in Grands Crus must be limited to 3.3 tons per acre (45 hectoliters per hectare) with a 20% allowance for increased yields. There is no official regulation on the use of mechanical harvesting, but most Grand Cru producers prefer hand picking because human pickers tend to be more delicate with the grapes and can distinguish better between ripe and unripe bunches. Over the rest of the Chablis region, mechanical harvesting was used by around 80% of the vineyards at the turn of the 21st century. The traditional style of vine training in Chablis is to have the vine trained low to the ground for warmth with four cordons stretching out sideways from the trunk.