The Internal Revenue Service considers wedded taxpayers married if they are legally married under state law, live together in a state-recognized common-law marriage, or are separated but have no separation maintenance or final divorce decree as of the end of the tax year.
Of the 56 million tax returns married couples filed in 2009, the latest year for which the IRS has published statistics (at the time of writing), 4.3 percent belonged to twosomes who filed separately. These partners reported individual income and expenses on individual tax returns. They had to agree on either itemizing expenses or using the standard deduction. By filing separately, their similar incomes, miscellaneous deductions or medical expenses likely helped them save taxes.