Previous year defined as the financial year preceding assessment, with special rules for newly established businesses and sources. The statute defines 'previous year' as the financial year immediately preceding the assessment year, with a special rule that where a business or source of income is newly set up during that financial year the previous year runs from the date of commencement to the end of that financial year. Historical and transitional provisions previously allowed adoption of an accounting period ending within the financial year, Board determined periods for particular persons or classes, distinct previous years for separate income sources, and restrictions on varying an adopted previous year without tax authority consent.