Set is the act of placing a card (Monster, Spell or Trap cards) on the field face-down.
Setting a Monster CardEdit
When you set a Monster Card, you place it in a Monster Card Zone horizontally and face-down. If the monster is Level 5 or 6, you must Tribute another monster first, then Set the Level 5 or 6 Monster Card (you do not need to show your opponent the card you Set). If the card has a higher Level (7+), the same procedure applies, but you must Tribute 2 Monster Cards from your side of the field first (unless there is a specific Tribute requirement). You can only set one monster per turn without the effect of another card, just as you can only normal summon once per turn without the effect of a different card.
Monsters may only be in face-down attack position by the effect of a card such as Darkness Approaches. That monster card is face-down and is placed vertically.
Setting a Spell or Trap CardEdit
When you Set a Spell/Trap Card, you place it on the field vertically and face-down, in a Spell & Trap Card Zone. Quick-Play Spell Cards cannot be activated in the same turn they are Set. However, you can activate any other type of Spell Cards in the same turn they were Set. Trap Cards however, cannot be activated in the same turn they were Set, unless a card's text designates otherwise, such as "Cathedral of Nobles".
Types of Monster Cards
Types of Spell Cards
Types of Trap Cards
Other Types of Card
ATK • DEF • Level • Attribute • Type
Playing Field Zones
Types of Summon
Types of Monster Effects
Types of Effect Monsters
Types of Spell Speed
Aqua • Beast • Beast-Warrior • Dinosaur