starting position to an extended position once assembled the stop openings 123 may be slightly wider than the width of the stops sw such that the stops 113 fit in the stop openings 123 in the assembled structure the stop openings 123 can be designed to define the starting and extended positions of the sliders 110 such that an inner surface defines the starting position and an outer surface defines the extend position the fixed pins 121 may be used as a quick reference to measure center to center distances of front posts on a rack the standard center to center distance is 18 312 inches as described above therefore the fixed pins 121 may have a pin distance pd between the centers of the pins 121 equal to about 18 312 inches the fixed pins 121 can be used as a quick u201c go no go u201d tool to determine whether the front posts are properly aligned now referring to fig1 and 13 the assembled structure is illustrated at the starting position fig1 and the extended position fig1 fig1 includes a top view 1 and a cross sectional front view 2 of the assembled structure the cross sectional view 2 is taken along section line a a at the starting position the sliders 110 are completely retracted towards the center of the structure 100 such that the stops 113 are in contact with the inner surface of the stop openings 123 at the starting position the structure 100 has a reference distance rd between the outside surfaces of the pin holders 111 equal to about 20 1 inches at the extended position the sliders 110 are completely extended outward from the center of the structure 100 in the horizontal direction such that the stops.