There are many different versions of the Flat - Topped Arbor.
This style has a doubled up 2" x 8" horizontal top members on front and back post sets. There is a 2" x 8" brace between front and back posts (though this varies upon application). The top, consists of 3/4" x 1" strips forming a box pattern (size and shape of material and boxes can be varied).
The sides are constructed of (2) - standard panel lattice sections. The lattice used ia 3/4" set inside a 3/4" x 1" channel surrounded by a 2' x 4" frame.