Xyz Materials (pronounced ik-seez) (Japanese: Ｘ（エクシーズ）素（そ）材（ざい） Ekushīzu Sozai, エクシーズ素（そ）材（ざい） in card text before Duelist Alliance), are cards stacked underneath an Xyz Monster. They are known as Overlay Units (オーバーレイ・ユニット, Ōbārei Yunitto), abbreviated ORU, in the anime and manga. When an Xyz Monster is Xyz Summoned, the monsters used for the Xyz Summon are stacked underneath the Summoned Xyz Monster. The attached Xyz Materials do not occupy a Zone.