Four square

The four square cipher is similar to the playfair cipher in its construction. Two '5x5' grids are created with mixed alphabets. These are placed at diagonal opposites of four squares. For the following example the keys of 'key number one' and 'number two key' shall be used.

               (1st cipher text)
(1st clear text)
  A B C D E       K E Y N U
  F G H I K       M B R O A
  L M N O P       C D F G H
  Q R S T U       I L P Q S
  V W X Y Z       T V W X Z

  N U M B E       A B C D E
  R T W O K       F G H I K
  Y A C D F       L M N O P
  G H I L P       Q R S T U
  Q S V X Z       V W X Y Z
(2nd cipher text)
                (2nd clear text)

The plain text is then split into groups of two letters. Nulls or some particular character may be added to make the pairings work. The message "how about dinner tonight" becomes:
HO WA BO UT DI NN ER TO NI GH TX

Next, the two letters are located in the clear text boxes. The original characters are in bold, the cypher in italic.

  . . . . . | . . . . .
  . . H ----------- O .
  . . | . . | . . . | .
  . . | . . | . . . | .
  . . | . . | . . . | .
------|-----+-------|------
  . . | . . | . . . | .
  . . | . . | . . . | .
  . . C ----------- O .
  . . . . . | . . . . .
  . . . . . | . . . . .

The first plain text becomes the character in the first cipher text, and the second plain text becomes the character in the second cipher text. The fully enciphered stream is:
OC TU NA QL NO FC EL QL GW RT PX

Of course, this can also be extended to a 6x6 block with A-Z and 0-9 and 10x10 block to encode numeric data.

Summertime school game I played during high school in Australia. It is somewhat different to the descriptiion given by ioctl.

The Set up

The court is made up of four equal sized squares or squarish rectangles. One person to a square, and if there are more than four players, then the others line up, waiting for their turn to enter the court. The game is played with a tennis ball on a concrete surface. The four squares are king (top left), queen (top right), jack (bottom right), and dunce or baby (bottom left). Badminton courts make the ideal court, however more advanced players like smaller courts since they pose more of a challenge.

The rules

The king serves by bouncing the ball such that it bounces in her or his own square and then bounces inside an opponent's square. It is required for all players, whether they are serving or merely passing the ball, that when it leaves their hand, it bounces once in their own square and then goes to an opponent's square. If the ball is entering your square, you do not have to let it bounce before you hit it, but if you do let it bounce, it can only bounce once. If you let the ball bounce twice or more in your square, you lose the rally. This is referred to as a double bounce. If you hit the ball into an opponent's square directly without letting it bounce in your own square first, you also lose. This is referred to as a fool. If you hit it out of bounds, you're also out. If you go out, then everyone preceding you, moves up a space. So if you were king, and you lost, then queen would move to king, jack to queen, baby/dunce to jack and the next person in the queue would move to baby. If there are only four players, then the person who went out, goes to baby/dunce.

Other rules

These rules are somewhat standard, although they can be modified or just ditched completely depending on those involved. Democracy is key here...if you don?t like how the game is run...suggest a change in the rules.

Faults:

Generally accepted to be 2 or 3 faults, if the king makes a bad serve (bounces twice in own square, etc.) the king gets 2 or 3 tries to get it right. When I played we ditched this rule since with 15 people to a square, 2 or 3 faults can be rather time consuming.

Liners:

This is a rule that is meant to settle the problem of whether the ball bounced out or just on the line. The idea is that in these sorts of cases, the players will agree on a liner, the person in the queen square, takes the ball, and serves it, except bounces it not in the queen's square, but rather on the cross formed by the four inner lines of the court. This rule again, is time consuming, but is sort of useless since replays (see below) are generally preferred.

Replay:

Sometimes in a game, a person may think someone went out, and no one can really agree on whether this is true or not. The players can decide on a replay which involves repeating the game, replicating the ball passes. So each player would have to remember who she or he passed the ball to. If you pass incorrectly, or break any of the other rules (passing the ball out of bounds for example), then you go out. This is often used instead of liners even though somewhat time consuming since no one knows what the hell is going on.

Challenge:

If two players have a dispute, then a challenge may be elected to solve the dispute. Essentially the king serves to one of the players (or if one of the challengees is the king, to his or her opponent) and the two players can only pass to each other until one of them loses.

Playon

This is an almost unused rule but the idea is that if one player makes a mistake, such as hitting the ball out, or fooling it, then if someone continues to play, they go out. This can actually go on for quite a while until someone realizes that someone made a mistake, and calls out playon. The last person to hit the ball is deemed to go out.

So who is the Winner?

Interestingly enough there really isn't anyway to judge who the winner of the game actually is. The standard way is that whoever is in king's position at the end of the session is deemed the winner. But this is arbitrary at best, and no one really cares who the winner is. Most people just decide whether they played well themselves personally.

Techniques

Low-ball:

By far the favorite tactic, is to try and ensure that the ball is as low and fast as possible, without make the ball double bounce, go out or fool or roll. A roll is RIGHT OUT. =). It takes a bit of practice but it is definitely worth it, and essentially this is the work horse of the game.

Body-shots:

Basically try and hit your the opponent with the ball as you pass it. This is I guess possibly physically dangerous, but since so far I haven't heard of anyone getting hurt, and it's always funny and uber-cool, it's still allowed. Most of the time the ball will double bounce and the person who got hit will go out since if it touches your body it counts as a hit. Note that since you can hit the ball with any part of your body you can occasionally be able to return this type of shot. The best idea however is usually to duck or step to the side and watch as the ball goes out.

Shifty shots:

Pass the ball through your legs or shots that look as if they're passing to a certain square when in fact they are going into another.

Steals There's nothing stopping you from hitting a ball from mid air, so you can steal the ball from going to a certain person.

Variations:

2-square, 3-square,6-square,8-square

2-square is just one person against each other, either each player taking half the court, or both players only taking one square and the other squares being set as out. Three square can involve the king taking the king and queen squares. Being the king here is harder since you have twice the area and two opponents. Bigger courts are just to accomodate more people. I have played on a 10-square court, which gets a tad boring since the action tends to centre around 4 squares anyhow. I have not tried the idea of having two kings, two balls etc but it sounds interesting.

Teams: You can of course have two or more teams playing. You can even have more than 4 people playing on a team. If you have 4 people in a team then you have to end up killing off one of your own. A rule that is useful in team games is that the king must serve to an opposing team unless there is no opposing team on the court. This is to prevent the king serving ideal shots to teammates to easily kill opponents. Occasionally in a normal game, two or more players may conspire to team up, and then wipe out independents.

Four square addendums:

In general, the ball cannot be "held", for much time. It can be caught but must move after about five seconds.

The square grid usually looks like this, and players rotate counter-clockwise from pawn to king. Players waiting to go "in" stand on the X.

X
_____________
|pawn |   KING    |
|_____|_______|
|  jack  | queen    |
|_____|_______|

 A player is usually "out" if the ball bounces twice (or three times, at the discretion of the group) within a player's square, or if an opponent throws the ball so it lands in the player's square but they cannot reach it to return it. You can get yourself out by throwing out of bounds.

"Moves" or "plays" are popular in four square. Some are a one-time move, others are chains.  You have to call moves--so basically if you do anything but just throw the ball normally, you have to yell it out. If you call a move, but mess it up, YOU are "out"--even if the person you threw it to didn't return the ball.

single-use:
Ace: ball slammed in far corner of square, kind of a underhand sideswipe
toilet paper: thrower holds the ball against his or her chest, under chin, on the flat part of both arms, and rolls it down arms and off hands. The 'unrolling' move is where the name is from.
giant: throw the ball REALLY hard so it bounces over the other player's head.
typewriter: the ball is dribbled repeatedly in your opponent's square, maxing them out on bounces. Really easy to fumble!

chains
popcorn: throw the ball to anyone. They must catch it, toss it in the air, and clap their hands once and yell "one" before catching the ball again, then throw the ball on. The next player has to clap and count "two", and so on.around the world: the ball must be thrown to the adjacent player, and continue around the square. This MUST continue in a circle until someone breaks it.death/deathmatch: the caller throws the ball to any player. These two must continue back and forth, not passing the ball to any other players, until one is eliminated

This is seriously one of the most fun school yard games to play as an adult (Red Rover and Manhunt are also really fun). A few rules Wuukiee left out:
The King serves the ball after someone gets out.
The King can make up rules (and like any good dictatorship, piss off people enough they'll all just try and kill only you).
You can't get out on a serve (keeps the King from spiking ball at people).
Usually people play that you can only hit the ball underhanded to keep people from spiking the ball.

Some other Foursquare lingo and rules:
Shoe shine: When someone hits the ball at your feet, and unless you're standing out of bounds it's an automatic out.
Inside-out, Outside-in: If the ball lands on an inside line whoever hit it last is out, but outside lines still count as your square.
Everytime you hit the ball you must say a word in a subject. If the King chooses "vegetables" then you have to say a vegetable without saying one that has been repeated before (if you can't, you're out).