The document discusses accounting periods and methods for partnerships and S corporations. It states that a partnership's tax year must end on the tax year of the majority interest partners, or if no majority, the principal partners. If there is no majority or principal partners, the tax year that results in the least aggregate deferral of income is used. Exceptions allow other fiscal year ends if valid business purposes or section 444 elections are met. The document also discusses short tax years, accounting methods, and provisions that mitigate the effects of an arbitrary accounting period.