Since companies are set up and operated over an indefinite life time comparisons over time can only be made by dividing the company's activities into arbitrary time periods. For external financial reporting the period is almost invariably a year, although for internal purposes, and interim reporting the periods chosen are often much shorter (arguably contributing to an inappropriately short-term decision emphasis in many companies).
With the need to provide comparable information in successive years in mind, it is necessary to adopt consistent ways to decide which transactions affect one year, and which the next: we must match the transactions to the years. This classification decision is controlled by conventional accounting rules similar to those applied in drawing up the balance sheet (in fact all the accounting rules now interact and affect the presentation of both statements).

