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View of the upper portion of the exterior of the theater. Focus on one of the upper windows. Note the protrusions, one below and one above the window. The upper protrusion has a hole in it and a large long pole was lowered through it and secured in the lower protrusion.
There were a series of poles all around the theater and from the top of them a canvas canopy was hoisted (sometimes by sailors) so as to provide shade for those who were attending the theater.
This cover is called a velum.
For a map and brief discussion on the significance of Aspendos Click Here.