In 2007, Bronstein (plaintiff) and her father-in-law purchased property as joint tenants for $1.35 million, subject to a mortgage. Bronstein’s husband had no ownership interest in the property. Both Bronstein and her husband used the property as their residence. Bronstein alone made the mortgage payments in 2007; neither Bronstein’s husband nor her father-in-law contributed to the payments. Bronstein and her husband filed married-but-separate federal income tax returns for 2007. Bronstein claimed a qualified residence mortgage interest deduction based on the $1 million acquisition indebtedness limit under 26 U.S.C. § 163. Neither Bronstein’s husband nor her father-in-law claimed a deduction for mortgage interest paid on the property. The Internal Revenue Service (IRS) (defendant) disallowed a portion of Bronstein’s deduction on the ground the deduction exceeded the $1 million acquisition indebtedness limit under § 163. Specifically, the IRS found that the $1 million acquisition indebtedness limit must be split among married couples who live in the residence and file taxes separately. The IRS concluded that the $1 million acquisition indebtedness limit cannot be claimed in full by one spouse. Bronstein appealed.