What is the sweep angle of the horizontal stabilizers on a Boeing 787 Dreamliner? Does it have to be equal to that of the wings, and how is it determined?
Simple image analysis shows it has higher sweep.
The general rules of empennage design can be found here (Hamburg University of Applied Sciences).
The sweep of the horizontal tailplane should be approximately 5° larger than the sweep of the wing. Thus a higher critical Mach number of the horizontal tailplane can be achieved and a loss of efficiency due to shock waves is avoided.
In addition, the lift gradient of the horizontal tailplane can be less than the lift gradient of the wing due to the increased sweep, so that the horizontal tailplane only reaches the stall state at larger angles of attack than the wing.