|Japanese (base text)||
The properties of a card (name, Attribute, Type, Level/Rank, ATK, DEF, effect) are not part of public knowledge while that card is face-down. A face-down card being looked at or revealed is still considered face-down (although its properties can be determined by card effects while being revealed) and is returned to its original position once the player(s) have finished viewing it.
In Yu-Gi-Oh! ARC-V, small orbs with glowing red eyes appear on top of face-down monsters.
In the Decks
Face-down cards in the Main Deck cannot be looked at by either player, except by a card effect. Face-down cards in the Extra Deck can be looked at by their possessor at any time, but cannot be looked at by their opponent.
On the field
Monster Cards can be Normal Set in face-down Defense Position (it is not possible for monsters to be in face-down Attack Position). Prior to its functional erratum at the start of Yu-Gi-Oh! VRAINS, "Darkness Approaches" could cause a monster to be placed in face-down Attack Position; additionally, the anime-only "Miraculous Hats" allows monsters to be in face-down Attack Position.
Spell and Trap Cards can be Set face-down in the Spell & Trap Zones or the Field Zone (in the case of Field Spell Cards). Quick-Play Spell Cards and Trap Cards cannot be activated during the turn they are Set. It is not possible to Set Pendulum Monsters face-down in Pendulum Zones.
Some card effects can banish cards face-down. Cards that are banished face-down can be looked at by their possessor at any time, but cannot be looked at by their opponent. However, the number of banished cards (including face-down banished cards) is public knowledge.