Solid 1 1/4″ square, 14-gauge steel construction with two removable bars instead of fixed bars. The removable bars allow the customers to STACK mattresses while lying down instead of the upright use of moving the mattresses on the fixed bar carts.  Allows up to (well how high can you stack and not cause an accident?!)—we recommend 4-5, but have seen upwards of 10 stacked on these carts.