Edge sorting is an advantage gambling technique in which players use subtle differences on the back of certain brands of cards to work out the face value of cards in the deck.
Players can establish whether or not a face-down playing card is high or low by looking for subtle differences in the pattern on the back.
These players then persuade dealers to unwittingly sort the cards into high and low values.
In most packs, all cards have identical backings, but some brands have unintentional irregularities. This means the two long edges of the card are consistently different and the pattern is asymmetrical on a 180-degree rotation.
Players ask dealers to rotate the face-down cards, saying they are superstitious.