In much anticipated guidance, the IRS has provided its first set of rules applying the Supreme Court’s ruling in United States v. Windsor. Under Rev. Rul. 2013-17, a same-sex couple who is legally married according to the laws of a jurisdiction that recognizes same-sex marriages will be recognized as married for all federal tax purposes regardless of where the couple lives. The revenue ruling provides, however, that a couple who enters into a registered domestic partnership, civil union or other formal relationship that is not a marriage under state law is not treated as married for federal tax purposes. The IRS also issued a series of Frequently Asked Questions (FAQs) providing some specific guidance on employee benefits and other issues. The guidance applies prospectively as of September 16, 2013. Taxpayers can also rely on the guidance retroactively for purposes of filing original, amended or adjusted federal tax returns, or claims for refund for any overpayment of tax, for open tax years.