Per IRS Publication 946 How to Depreciate Property, on page 3:
Depreciation is an annual income tax deduction that allows you to recover the cost or other basis of certain property over the time you use the property. It is an allowance for the wear and tear, deterioration, or obsolescence of the property.
Per IRS Topic No. 704 Depreciation:
Depreciable or Not Depreciable
The kinds of property that you can depreciate include machinery, equipment, buildings, vehicles, and furniture. You can't claim depreciation on property held for personal purposes. If you use property, such as a car, for both business or investment and personal purposes, you can depreciate only the business or investment use portion. Land is never depreciable, although buildings and certain land improvements may be.
You may depreciate property that meets all the following requirements:
Land is not depreciable.
The Modified Accelerated Cost Recovery System (MACRS is used to recover the basis of most business and investment property placed in service after 1986. Under MACRS, the cost of qualified property is expensed over predetermined periods.
The Accelerated Cost Recovery System (ACRS) is the method of depreciation used to expense the cost of assets placed in service before 1987. IRS Publication 534 Depreciating Property Placed in Service Before 1987 explains this method of depreciation.
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