The purpose of shuffling a deck of cards is to mix up the order of which the cards are in. Card shufflers have many different ways of doing this. There are several different methods people use to mix cards thoroughly. From spreading the cards out on a table and randomly shoving the cards back together into a mixed deck to precisely performing a perfect riffle shuffle, the technique used often depends on the experience of the shufflers.
The most common method used in the United States is the riffle shuffle. This is when the deck is cut in half and the cards are held with half in the left hand and half in the right with the thumbs holding the corners of the top of the cards while the index fingers apply pressure to the top of the decks. The shufflers thumbs are pulled up slightly while the index fingers apply pressure resulting in cards being released from both decks and overlapping each other slightly so that when all cards have been released, the two decks can be pushed together into one again.
In Europe, the riffle shuffle is considered fancy and hard to maneuver. So the most common method used there is the overhand shuffle. For the overhand shuffle, card shufflers hold the entire deck of cards in their right hand. The thumb is on the short side of the deck closest to the shuffler with the index finger resting on the long side at the top. The other three fingers are holding the other short side so that the deck is secure. Shufflers then use their left thumb to pull a group of cards into the left hand while the remainder of the deck stays in the right hand. This process is repeated until the entire deck is transferred over to the left hand. This method takes a little bit longer than the riffle shuffle as it requires more work and effort to get the cards thoroughly mixed.
It was reported by Professor Persi Diaconis and his team in 1990 that in order to shuffle a standard deck of 52 cards so that it is perfectly randomized, it would take 7 riffle shuffles. Other studies performed had estimated it being between 5 and 20 times. Basically, if you shuffle a deck less than 7 times, some clusters of cards can stay together. On the other hand, his study found that shuffling the cards more than 7 times have little impact. Seven seems to be the lucky number. This same study found that using the overhand shuffle, you would need to shuffle the deck 2500 times to accomplish complete randomization. That’s a lot of shuffling! Card shufflers would most likely get paid more if they were required to shuffle that many times.
There are other types of shuffling that can be seen throughout the world. Some card shufflers use the Faro shuffle method while some use the Hindu shuffle. Some use the Riffle shuffle while some use the Overhand shuffle. Some even use a variety of different methods. When it comes down to it, the trick to randomizing a deck of cards is repetitiveness. Generally speaking, many card shufflers don’t realize how many times they need to shuffle a deck of cards in order to achieve a thorough mix.