Our Medium-Heavy Duty Cantilever Rack Adder Kits come with 1 tower that is single or double sided, 1 or 2 cantilever rack bases depending on whether your system is single or double sided, and the desired brace kit for achieving the tower spacing you need for your particular products and the corresponding cantilever arms. These are offered in all the same dimensions and capacities that the starter units come in. When building your storage system you simply start by choosing the appropriate cantilever rack starter unit that works best for your storage application. Then choose the corresponding adder unit as you continue to build your storage system. The first adder unit will bolt to your starter unit. Each additional adder unit will bolt to the previous adder unit until you have fulfilled your storage requirements.