National base areas are defined on the basis of the average of areas planted to cereals, oilseeds and protein crops between 1989 and 1991. The sum of individual areas claimed for payments — areas under set-aside and areas planted in cereals, oilseeds and protein crops — cannot exceed the national base area. If exceeded, there is a reduction in area payments and a penalty land set- aside which increases the level of mandatory set- aside during the following year.