The historical cost is then depreciated: it is systematically reduced to the recoverable amount, over the estimated useful life of the asset, to reflect the asset's usage. The depreciation (reduction of historical cost) is charged to expense. In most cases the "straight line" depreciation method is used, resulting in the same depreciation charge each year until it is expected to be sold or no further economic benefits obtained from it. Other patterns of depreciation are used if assets are used proportionately more in some periods than others.