One of the many questions surrounding Leonardo’s Mona Lisa concerns the landscape visible in the portrait’s background: Does it depict an imagination of Leonardo’s mind, a real world landscape or the motif of a plane canvas that hung in Leonardo’s studio, behind the sitter? By analyzing divergences between the Mona Lisa and her Prado double that was painted in parallel but from another perspective we found mathematical evidence for the motif-canvas hypothesis: The landscape in the Prado version is 10% increased but otherwise nearly identical with the Louvre one, which indicates both painters used the same plane motif-canvas as reference.