Assembling the elevators.
As you can see they can be rotated over a shaft, just like the real one does. The Dc-9 doesn't have full elevators, but stabilizers and elevators. The detail is that the trim of the plane is achieved tilting the stabilizer instead of the tabs of the elevators.
And then, why the Dc-9 elevators has tabs? Well...
Why Are MD80 Elevators Split? - Paperblog
Anyway, tonight or tomorrow I'll finish the tail section and proceed to design the wings and wing roots.