Composition Materials (Japanese: (こう)()()(ざい) Kōzu Sozai) are monsters used in the Composition Summoning of a Composition Monster. A limit is almost always set on them, and usually a minimum of 4 is required. When a Composition Monster is Summoned, the Composition Materials are stacked in an invisible "zone" in front of the Composition Monster, with the Base Monster on the bottom of the stack.

Like Xyz Materials, Composition Materials are not counted as monsters or as being on the field, since their placement spot is not physically part of the field.  Composition Materials have two states: "active" and "inactive".  Active Composition Materials are face-up in the stack.  When a Composition Monster is Summoned, all its Materials are active.  Most Composition Monster effects require you to "deactivate" a Material: To do this, simply flip the chosen Material face-down.  Until the Composition Monster leaves the field, the stack of Materials is not sent to the Graveyard, as card effects can "reactivate" Materials.  Also, once a stack of Composition Materials is placed, Materials cannot be added to or removed from the stack. Unlike Xyz Materials, Composition Materials are not attached to their respective Composition Monster, but they do belong to them, so effects must be worded to indicate a Composition Monster's ownership of its Materials.

In the anime, Composition Materials are called Soul Towers, and all Duel Disks made in that time have a special slot for them underneath the Spell and Trap Card slots.  The Soul Towers are formed by the Materials transforming into them, and they appear around the Composition Monster as special spires with white crystals on top, which connect to the monster with streaks of colored lightning matching the Attribute of the monster.  The abbreviation for the active Soul Towers in the AR Vision is "AST".  When a Soul Tower is deactivated, the crystal turns black and the lightning ceases.

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