They were a type of monster card that you needed 'tuners' to summon..! Most people use these though! [He holds up a black-rimmed monster card.] Xyz..! Using the right number of monsters of the same level, you can summon an Xyz monster, with the other monsters becoming its overlay units..!

First you have to summon a normal monster though. Like this. [This monster card has an orange rim.] Anything that's four or less levels can be summoned normally. Anything higher needs a tribute though- if it's level five or six, you need one tribute. If it's higher than that, you need two..! [He might have mixed his facts up for anything higher than eight levels though, but shhhhh.]