The site of Yaxchilán (Chiapas, Mexico) is one of the best-known Classic Maya cities, despite relatively little stratigraphic excavation. The dynastic history of the site is recorded in numerous legible inscriptions and was worked out more than 30 years ago by Tatiana Proskouriakoff (1963, 1964) in her epoch-making historical interpretations. A historical puzzle left by Proskouriakoff and later investigators is a 10-year gap (A.D. 742-752) between the death of a Late Classic king, Shield Jaguar II, and the accession of his successor and son, Bird Jaguar IV. Various explanations have been offered for this interregnum, often suggesting a rival heir to the throne, as Bird Jaguar was the son of his father's late, secondary, foreign wife. Nevertheless, no evidence has been put forth concerning a child by the king and his earlier, primary wife, a woman known as Lady Xok. This paper reexamines the epigraphic evidence by applying what is now known about Classic Maya grammar and the canons of Classic Maya literature to an inscription on the house of Lady Xok. Viewed from this perspective, a well-known inscription yields the name of the missing heir, and other evidence suggests the nature of his fate and the reason he did not take the throne.