Set

Set is the act of placing a card (Monster, Spell or Trap card) on the field face-down.

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Setting a Spell or Trap Card

This is what a set Spell or Trap Card looks like.

When you Set a Spell/Trap Card, you place it on a Spell & Trap Card Zone face-down in a vertical position, except Field Spell Cards, which can only be placed on the Field Card Zone, even if it is face-down.

You can activate any type of Spell Cards in the same turn they are Set except for Quick-Play Spell Cards. Trap Cards can't be activated on the same turn they were Set, unless by the effects of other cards (such as "Temple of the Kings").