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.