A document published by the Internal Revenue Service (IRS) that provides information to taxpayers who are married, live in a state supporting community property laws and are filing separate tax returns. Income and property is considered community if it is purchased or owned while the couple is married. All else is considered separate. Couples filing separately report half of their joint income and all of their separate income.
Not all states treat community property the same way. IRS Publication 555 only addresses Arizona, California, Idaho, Louisiana, Nevada, New Mexico, Texas, Washington and Wisconsin. If the tax payer's primary residence is in one of these states, or if residency has been established by voting or by paying state income tax, then community property rules are likely to apply.
Married taxpayers will typically have a lower tax obligation if a joint return is filed rather than an inpidual return, but this is not always the case.