Adder decks are used to expand the number of poles on a relay. The basic 4-pole relay can be easily converted to an 8 pole or 12 pole relay by installing one or two adder decks. The class 8501 type XB20 comes with 2 convertible contact cartridges and will accept 2 additional convertible contact cartridges. The same type XB adder deck is used for both the middle and upper decks of the AC or DC relay.