A spouse who unknowingly filed a joint return with their spouse who had reported an understatement of tax due to erroneous items. The unknowing spouse must prove that at the time the tax return was signed he/she did not know, or have reason to know, there was an understatement of tax. Also with the fact and circumstances taken into consideration, it must show that it would be unfair to hold the unknowing (innocent) spouse liable for the understatement of tax. To request innocent spouse relief, the taxpayer must file Form 8857. (See also Equitable Relief and Separation of Liability).