Speed (also called Spit) has no turns - both players play at the same time, as fast as they can, until one of them is out of cards. You play a card that is one rank above or below whatever is showing in the middle. A game lasts about 3 minutes and is usually played best of several.

What You Need

Players: Exactly 2.
Deck: One standard 52-card deck. Suits do not matter at all.
Extras: A table with room for six piles.

Key Terms

Draw pile: Your personal face-down stack of 15 cards.
Hand: The 5 cards you hold and play from.
Replacement piles: Two 5-card stacks between the players, used to break a deadlock.
Play piles: The two face-up cards in the very middle that everyone plays onto.
In sequence: One rank higher or lower than the card showing. Suit is irrelevant.

Setup

Split the deck and deal each player 20 cards: 15 face down as their draw pile, and 5 in hand.
Place the remaining 12 cards as two replacement piles of 5, one on each side of the middle, and 2 single cards face down in the centre between them.
When both players are ready, flip the two centre cards face up at the same time. Play begins immediately.

How to Play

There are no turns. Both players play whenever they can:

Play a card from your hand onto either centre pile if it is one rank higher or lower than the card showing there. A 7 accepts a 6 or an 8.
Aces wrap around, so a king can go on an ace and an ace can go on a king.
Immediately draw from your draw pile to bring your hand back up to 5 cards. You may only hold 5 at a time.
If both players get completely stuck, both say so, then each flips the top card of their replacement pile onto their centre pile at the same time. Play restarts.

Scoring & Winning

The first player to get rid of both their draw pile and their hand wins the round.
There is no scoring beyond that. Most people play a set of rounds and count wins.
If both players are stuck and the replacement piles are gone, shuffle the centre piles, split them into new replacement piles, and continue.

Example

The centre card is a 7 and you hold a 6. You slap it down, immediately draw a replacement card, and look for the next play while your opponent is still deciding.

Variations

Spit: the more common name outside the US, with slightly different pile sizes and a scramble for the smaller pile at the end.
California Speed: cards are laid out in a grid and players slap matching pairs instead.
Three or four players: possible with a second deck and extra centre piles.

Tips for New Players

Play with both hands. Nothing stops you.
Keep your five hand cards spread out and facing you so you can see every option at a glance.
Watch both centre piles, not one. Beginners fixate on the pile in front of them and miss easy plays.