"Above the scene of the historical ruler engaged in sacrifice is a Skyband Monster. This symbol has a local meaning which may differ from the meaning of the skyband at other cities. Here it indicates the royal role in maintaining the paths of the sun, moon, and planets thought the sky. The myth of the Hero Twins is evoked by the use of nine GI or GIII heads pendant from the skyband. Two ancestor cartouches ride the skyband. Between them is a bust of GI or GIII. The portrayal of the ruler's ancestors in the cartouches is at once genealogical documentation, a depiction of filial devotion, and reference to the cosmic role of the deceased royal lineage as embodiment of the sun and moon."
Carolyn E. Tate, Yaxchilan: The Design of a Maya Ceremonial City, p. 100
NOTE: Although they are hard to see, the "Ancestor Cartouches" are on either side of the face of GI/GIII at the top of the stela. The ancestors are both sitting cross-legged and are holding small bicephalic skybands.