"What is the fair market value of an object that cannot be sold?"

"The object under discussion is 'Canyon,' a masterwork of 20th-century art created by Robert Rauschenberg that Mrs. Sonnabend’s children inherited when she died in 2007."

Because the work, a sculptural combine, includes a stuffed bald eagle, a bird under federal protection, the heirs would be committing a felony if they ever tried to sell it. So their appraisers have valued the work at zero.

 But the Internal Revenue Service takes a different view. It has appraised “Canyon” at $65 million and is demanding that the owners pay $29.2 million in taxes.

“It’s hard for me to see how this could be valued this way because it’s illegal to sell it,” said Patti S. Spencer, a lawyer who specializes in trusts and estates but has no role in the case.