26 CFR §1.167(b)-2

1. (a)Application of method. Under the declining balance method a uniform rate is applied each year to the unrecovered cost or other basis of the property. The unrecovered cost or other basis is the basis provided by section 167(g), adjusted for depreciation previously allowed or allowable, and for all other adjustments provided by section 1016 and other applicable provisions of law. The declining balance rate may be determined without resort to formula. Such rate determined under section 167(b)(2) shall not exceed twice the appropriate straight line rate computed without adjustment for salvage. While salvage is not taken into account in determining the annual allowances under this method, in no event shall an asset (or an account) be depreciated below a reasonable salvage value. However, see section 167(f) and § 1.167(f)-1 for rules which permit a reduction in the amount of salvage value to be taken into account for certain personal property acquired after October 16, 1962. Also, see section 167(c) and § 1.167(c)-1 for restrictions on the use of the declining balance method.
2. (b)Illustrations. The declining balance method is illustrated by the following examples:
3. (c)Change in estimated useful life. In the declining balance method when a change is justified in the useful life estimated for an account, subsequent computations shall be made as though the revised useful life had been originally estimated. For example, assume that an account has an estimated useful life of ten years and that a declining balance rate of 20 percent is applicable. If, at the end of the sixth year, it is determined that the remaining useful life of the account is six years, computations shall be made as though the estimated useful life was originally determined as twelve years. Accordingly, the applicable depreciation rate will be 16 2/3 percent. This rate is thereafter applied to the unrecovered cost or other basis.