a. at least two spaced apart vertical posts being adaptable to be secured to a support surface;

b. at least two spaced apart upper and lower hollow horizontal rails each horizontal rail having an enclosed upper surface with a plurality of spaced apart square shaped apertures and an enclosed lower surface with a plurality of spaced apart square shaped apertures, a respective square shaped aperture in an upper surface aligned with a respective square shaped aperture in a lower surface;

c. means for attaching said at least two horizontal rails to said at least two vertical posts such that corresponding square shaped apertures on said at least two horizontal rails are aligned;

d. a plurality of spaced apart vertical pickets having a square shaped cross-section and having a hollow interior and respectively secured in mutually parallel relationship to said at least two horizontal rails, each said square shaped vertical picket having two opposite end portions respectively inserted into said aligned square shaped apertures of said at least two horizontal rails, each said end portion of each said square shaped vertical pickets having two oppositely disposed and aligned transverse apertures, each aperture having a locking tongue extending into the aperture;

e. means for locking said each end portion of said each square shaped vertical picket to one of said at least two horizontal rails when said end portion is inserted into a respective aligned set of square shaped corresponding apertures of said respective one of said at least two horizontal rails, said locking means each comprising a one-piece generally U-shaped spring clip having a bent middle portion and two opposite end portions each having an outwardly protruding stopper, each stopper having an upper ramp side having an opening which receives the locking tongue, a lower ramp side and a vertical transverse tip, each said spring clip respectively inserted into said each end portion of said each vertical picket with said opposite stoppers of said spring clip secured to the vertical picket by the locking tongue received within the opening in the upper ramp side, and each upper and lower ramp side respectively protruding through said two oppositely disposed and aligned transverse apertures of each said end portion of each said vertical picket; and

f. wherein when said each end portion of each said vertical picket is inserted into one of said corresponding aligned sets of square shaped apertures of said respective one of said horizontal rails, said stoppers of said spring clip are inserted in said each end portion of each said square shaped vertical pickets and extend through said transverse apertures of said each end portion of each said square shaped vertical pickets such that for the upper horizontal rail the locking tongue engages the opening in the upper ramp side of the stopper so that the lower ramp side of the stopper rests upon the interior of the lower surface of the horizontal rail and the transverse vertical tip is compressed against the interior wall of the square shaped vertical picket and for the lower horizontal rail the locking tongue engages the opening in the upper ramp side of the stopper so that the lower ramp side of the stopper rests underneath the interior of the upper surface of the horizontal rail and the transverse vertical tip is compressed against the interior wall of the square shaped vertical picket such that vertical movement of each said square shaped vertical picket relative to said respective one of said horizontal rails is prevented.