Monster Zones (モンスターゾーン Monsutā Zōn), formerly Monster Card Zones (モンスターカードゾーン Monsutā Kādo Zōn), are the zones where monsters are placed on the field. There are two types of Monster Zones: Main Monster Zones and Extra Monster Zones.
On game mats, Main Monster Zones are located in the upper-middle and are often colored brown. Extra Monster Zones are located in the center of the field and are colored blue.
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 Spell & Trap Zone). Likewise, Spell and Trap Cards in the Monster Zones are treated as monsters (although Trap Monsters are still 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.
A monster Summoned/Set from anywhere other than the Extra Deck can be Summoned/Set in any unoccupied Main Monster Zone on the appropriate player's field (unless the zone has been rendered unusable by a card effect). A monster Summoned from the Extra Deck can only be Summoned in an Extra Monster Zone or a Main Monster Zone that a Link Monster points to. Prior to the introduction of the Extra Monster Zones, monsters Summoned/Set could be Summoned/Set in any unoccupied Monster Zone on the appropriate player's field.
A monster cannot be Summoned unless there is an appropriate Monster Zone to Summon that monster in, unless performing its Summon would cause an appropriate Monster Zone to become available. For example, even if all of a player's Main Monster Zones are occupied, that player can still Tribute Summon a monster to their field as long as they Tribute at least one monster in their Main Monster Zones. Likewise, even if the opponent controls monsters in both Extra Monster Zones due to achieving an Extra Link, the player can still Summon a monster to an Extra Monster Zone as long as one of the Extra Monster Zones becomes available when performing the Summon (such as by using a monster in an Extra Monster Zone as material).
If control of a monster is changed or a temporarily banished monster returns to the field, it is placed in its controller's Main Monster Zone (regardless of its original zone). If there is no available Main Monster Zone, that monster is sent to the Graveyard by game mechanics.
The player who performs the Summon or Set, regardless of who will control the Summoned monster, decides which Monster Zone the monster is Summoned/Set in. The player must decide which zone the monster will be Summoned/Set in when they declare the Summon/Set, before the timing to negate the Summon.
Monsters cannot be moved between Monster Zones, except by a card effect such as "Senet Switch".