Assets are divided into classes by type of asset or by business in which the asset is used. (See tables of classes below.) Where a general class based on the nature of the asset applies (00.xx classes below), that class takes precedence over the use class. For each class, three lives are specified: one for regular depreciation (GDS in the tables below), one for the alternative depreciation system (ADS), and a class life. Taxpayers may be required to use ADS or otherwise may elect which of the three lives to use. Lives for personal property vary from 3 years to 20 years. Land improvements must be depreciated over 15 or 20 years. Other real property must be depreciated over 27.5 years for residential property, 39 years for business property, and 40 years under ADS. In addition, shorter lives are provided for certain property, including computers and peripheral equipment, certain high technology equipment, special purpose agricultural structures, and certain other items. MACRS also excluded certain property in regulated industries, as well as films, video tapes, and sound recordings, and certain other items. Also, leasehold improvements to realty are generally treated as real property under MACRS. Depreciation methods Only the declining balance method and straight line method of computing depreciation are allowed under MACRS. Taxpayers using the declining balance change to the straight line method at the point at which depreciation deductions are optimized. See the tables below. All tangible personal property acquired during the year is considered placed in service in the middle of the tax year ("half-year convention"). Real property is considered placed in service in the middle of the month in which acquired ("mid-month convention"). Special rules apply for pro rating deductions for short tax years and for the first year of business, or where more than 40% of tangible personal property additions are in the final quarter of the year. The method and life used in depreciating an asset is an accounting method, change of which requires IRS approval. Taxpayers may track the basis and accumulated depreciation of assets individually or in vintage accounts, as in the old ADR system. Where assets are tracked in vintage accounts, a first-in-first-out convention is usually applied to determine basis of assets retired. Special allowances and bonus depreciation At various times, additional depreciation deductions have been allowed to encourage investment. These allowances generally have had limitations. For example, an additional deduction of