26 USC 446 - General rule for methods of accounting

(a) General rule 
Taxable income shall be computed under the method of accounting on the basis of which the taxpayer regularly computes his income in keeping his books.
(b) Exceptions 
If no method of accounting has been regularly used by the taxpayer, or if the method used does not clearly reflect income, the computation of taxable income shall be made under such method as, in the opinion of the Secretary, does clearly reflect income.
(c) Permissible methods 
Subject to the provisions of subsections (a) and (b), a taxpayer may compute taxable income under any of the following methods of accounting
(1) the cash receipts and disbursements method;

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(2) an accrual method;
(3) any other method permitted by this chapter; or
(4) any combination of the foregoing methods permitted under regulations prescribed by the Secretary.
(d) Taxpayer engaged in more than one business 
A taxpayer engaged in more than one trade or business may, in computing taxable income, use a different method of accounting for each trade or business.
(e) Requirement respecting change of accounting method 
Except as otherwise expressly provided in this chapter, a taxpayer who changes the method of accounting on the basis of which he regularly computes his income in keeping his books shall, before computing his taxable income under the new method, secure the consent of the Secretary.
(f) Failure to request change of method of accounting 
If the taxpayer does not file with the Secretary a request to change the method of accounting, the absence of the consent of the Secretary to a change in the method of accounting shall not be taken into account
(1) to prevent the imposition of any penalty, or the addition of any amount to tax, under this title, or
(2) to diminish the amount of such penalty or addition to tax.