When using double declining balance

When using double declining balance method to calculate depreciation, because at the beginning of each year not deduct the book value of fixed assets, expected net salvage value, so in order to avoid the cold face value of the fixed assets has dropped below its expected net I value, usually two years prior to the expiry of the fixed number of year of the depreciation of fixed assets, book value of fixed assets that will be the balance after the deduction of the expected net salvage value, on average, amortization, namely using the straight-line method in the last two years.