Card-Trick-Corner
Observe the pattern / design of your playing cards deck. Compare the appearance before and after you turn it 180 degree. Observe the back side and the front side of your playing cards. Do you realize any differences when you turn it 180 degree?.
The secret behind the front side of playing cards. (the "face")
You were already knew that the face of each card carries markings that distinguish it from the other cards in the deck.
Figure 1.1
Figure 1.2 (after turn it 180 degree)
That difference also exist on other cards face. There are many cards design but much alike that difference are also exist on A,3,5,6,7,8,9 of Heart, Spade and Clover and also on 7 Diamond. And for some design that difference also exist on Jack, Queen, and King of spades.
The secret behind the back side of playing cards.
That difference also exist on some of back design / pattern of playing cards decks.
Figure 2.1
Figure 2.2 (after turn it 180 degree)
Actually the less obvious the difference that exist the better, because that also means the less chance your audience aware of it.
Let's take an example of how to use it on the simple "find the card" card trick. Prepare (arange) the cards in away that you can tell the difference if one of the cards being turned 180 degree (upside down). Then you ask your audience to choose one card (free choice).
When your audience already take the cards, tell your audience to see the card. While your audience see the card, slightly turn the cards you hold 180 degree (upside down).
Then tell your audience to return the card to the deck right in the middle or wherever he likes, then you can shuffle the deck (as long as you are not drop the cards and make the cards upside down) and then you can find the chosen card by looking "the upside down card".