Cards on the field are treated as monsters if and only if they are in a Monster Zone. Monster Cards on the field are not treated as monsters if they are in other Zones (such as the Pendulum Zone or Spell & Trap Zone). Likewise, Spell and Trap Cards in the Monster Zones are treated as monsters (although Trap Monsters are treated as Trap Cards as well, so they also require a Spell & Trap Zone but are placed in the Monster Zone). Xyz Materials are not considered to be in a Monster Zone.
If all 5 of a player's Monster Zones are occupied, then monsters cannot be Summoned to their side of the field, unless the monster is Summoned by removing monsters from the field. As such, a monster can be Summoned while all the Monster Zones are occupied by Tributing a monster(s) on that side of the field for a Tribute Summon or Ritual Summon, using a monster(s) on that side of the field as Material (Fusion, Synchro or Xyz), or by otherwise removing a monster(s) from the field for that Summoning procedure.
The player who performs the Summon or Normal Set, not the player who controls the Summoned monster, decides which Monster Zone the Summoned/Set monster is placed in. A monster can be Summoned/Set in any unoccupied Zone (unless the Zone has been rendered unusable by a card like "Ojama King"). The Zone into which a monster will be Summoned/Set is decided before the Summon negation window. Monsters cannot be moved between Zones, except by a card effect like "Senet Switch".
On game mats, they are located in the upper-middle and are often colored brown.
- ↑ "Master Rule 3 Overview". Konami. http://www.yugioh-card.com/hk/howto/master_rule_3.php?lang=cn-t.