Which diagram represents a line of symmetry of the regular octagon? An octagon has a vertical fold that goes through the left point of the top side to the left point on the bottom side. An octagon has a horizontal fold that goes through the middle of the octagon and splits the sides into 2 equal parts. An octagon has a diagonal fold that goes through the middle of the uppercase right side and the bottom corner of the left side. An octagon has a diagonal fold that goes through the lower point of the top right side to the left point of the bottom side.