Katia Popov and her husband (plaintiffs) lived with their child in a one-bedroom apartment. Popov was a professional violinist who performed with orchestras and made recordings. The recording studios offered no space and almost no time for practicing the music to be recorded. Therefore, musicians had to find their own practice spaces. Popov’s practice space was the living room of her small apartment. The living room was furnished and used solely by Popov as her practice space. Popov spent more time practicing at home than she spent performing. The Popovs deducted a portion of their apartment costs as home-office expenses. Under § 280A(c)(1)(A) of the federal tax code, home-office expenses were tax deductible only if the home office exclusively served as the taxpayer’s principal place of business. The commissioner of internal revenue (commissioner) (defendant) disallowed the deduction, determining that Popov’s principal places of business were the venues where she performed professionally, not the living room where she practiced. The Popovs petitioned the tax court for a redetermination, and the tax court ruled for the commissioner. Popov appealed.