GMC 2005 Sierra Denali Owner's Manual page 110

A. Back rail
B. Side rail
C. Rear bow
2. Remove the tonneau cover from the notch in the
front rail on the driver's side of the vehicle.
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D. Front bow
E. Front rail
3. Remove the two bows by gently pushing them
against the spring-loaded end. The two bows
are labeled front and rear to remind you of the order
in which to reinstall them.