The rulebook.

Two players, two fleets, two seas. Each hides their ships on their own sea – the other can't see them. Then you take turns firing at the other's sea, trying to find their ships.

Whoever sinks all of the opponent's ships first wins. There are no draws.

Read on about the cells, the fleet and how a game goes.

Each sea is a grid of 10 × 10 cells. The columns are called A to J, the rows 1 to 10. So every cell has a name: the letter first, then the number – like B7 or J10.

Here on the site you can click cells or type their name (key /). Lower case or with a space works too: b 7.

1. Placing: both place their fleet, each on their own (how). Shooting only starts once both are done.
2. Shooting: who starts is set when the game is created – you, your opponent or chance. Then you take turns, one shot each. Depending on the rules, a hit earns another shot.
3. End: as soon as one fleet is completely sunk, it's over (end of the game).

When you create a game you choose one of two fleets. It holds for both sides.

The number in brackets is the length: that's how many cells the ship covers. All ships are one cell wide.

Every ship lies in a straight line, across or down – never diagonally. It must lie entirely in the sea, and two ships can't lie on top of each other.

Whether ships may touch is set by the game (keeping your distance).

Once the fleet is in the water and both are ready, it stays where it is. Before that, “Change again” lets you rearrange everything. How placing works with mouse and keyboard is under placing on this site.

When creating the game you decide whether ships may touch. Without that – the default – every ship needs one cell of water all around it, diagonally too.

That has a practical consequence: once a ship is sunk, no other ship can be on any cell around it. With the help “mark water” the board shows you those cells.

If ships may touch, they can lie side by side. That makes searching harder: two hits next to each other may belong to two different ships.

Whoever's turn it is names a cell on the opponent's sea – here: click it or type it. The answer comes right away:

• Water – a miss. There's no ship here.
• Hit – there's a ship here.
• Sunk – that was the last intact cell of that ship (more).

Each cell can only be fired at once. On your own sea you see where your opponent has fired.

A ship is sunk when all its cells have been hit. That is always announced – so you know which ship went down and where it lay.

Here on the site a sunk enemy ship appears on their sea, and it is crossed out in the fleet overview. So you always know which ships are still missing.

A popular rule, and the default here: whoever hits may fire again right away – until a shot goes into the water. Only then is it the other's turn.

Without this rule you strictly take turns, hit or not. You decide when creating the game; the game info next to the board shows which applies.

The game is over as soon as the last cell of the last ship of a fleet has been hit. Whoever did that wins. Then both seas reveal everything.

A game also ends when someone resigns, misses the clock three times in a row in a timed game, or does nothing for 14 days in a correspondence game. There are no draws.

The smallest ship is two cells long. So it always covers one “light” and one “dark” cell of a checkerboard. That's why it's enough to fire at every other cell while searching – you'll still find every ship.

Once the destroyer is sunk and the smallest remaining ship is three cells long, it gets even better: every third cell in each row is enough.

After a hit it pays to try the four neighbouring cells: above, below, left, right. As soon as two hits lie next to each other you know the direction – then just keep going along that line until the ship sinks.

You never need to fire diagonally: ships don't lie diagonally. And if ships may not touch, the cells diagonally next to a hit are always water.

• No pattern: all ships across in one corner are found quickly. Mix the directions.
• Distance pays: if ships lie far apart, a sunk ship gives little away about the others.
• Edges are no secret: many think nobody looks at the edge – that's exactly why good players do.
• No plan? “Random” places the fleet for you, following the rules of the game.

Every game starts with setting up – a step of its own in which you only see your sea and your dock with all your ships. Everything else comes after; “Cancel” takes you back to the start. Click a ship, then the sea – it appears where the pointer is. Green means: fits. Red means: not possible, and with the help “explain” it also says why.

• R, a right click or “Rotate” turns the ship.
• Clicking a placed ship picks it up again.
• “Random” (Z) places the remaining ships, “Clear” (L) takes everything off.
• “Done” (F) sends the fleet into the water. While your opponent is still placing, “Change again” takes you back.

With the keyboard: a number for the ship, arrow keys to move, Enter puts it down.

The board is an opened game case: your sea at the bottom with your fleet, and in the lid the opponent's sea as a radar screen on which your shots land as pegs.

• D or “3D” switches to the flat view: the lid folds back until it lies flat, the camera looks down from above – and then the two grids lie one above the other, the enemy sea on top. This works while placing too; then the flat view shows only your sea.
• Your ships take turns firing: each shot comes from the next ship still afloat.
• Sunk enemy ships lie on the radar screen with their outlines; at the end you also see there all the ones you didn’t find.
• K or “Camera” pans between overview, enemy sea and your own sea.
• If you have set “reduce motion” in your system, the effects are shorter and calmer.

Without WebGL 2 there's only the flat view. Your browser remembers the choice.

When creating the game you choose which help it allows. It holds for both sides and the whole game:

• Mark water: cells around a sunk ship that can't hold anything any more get a small dot (only if ships may not touch, see keeping your distance).
• Explain what isn't possible: if a ship doesn't fit or a cell was already fired at, the game says why – with a link to this rulebook.
• Hints (against the computer only): the “Hint” button (T) marks the cell where a ship most likely is.

In the game, “Help” (H) hides the allowed ones for a while. Some badges are only for games without a hint.

The computer has five levels, from the deckhand who fires at random to the admiral who calculates every shot. You can change the level until the first shot; then it holds for the whole game, and exactly that level counts for your progress.

The computer doesn't cheat: it only sees what you see too – water, hits, sunk ships. The stronger levels also hide their fleet more cleverly.

In a timed game every shot has a time limit – 20 seconds, for example. There's no total time: the clock starts afresh with every shot.

• Miss the time and you lose the shot; it's the other's turn.
• Miss it three times in a row and you lose the game.
• Placing has two minutes (paper fleet: three). Whatever isn't placed by then is placed at random – what you had placed stays.

The clock keeps running on the server, even if someone leaves the page. The opponent sees that, and at the end it says how often and how long.

A correspondence game has no clock: everyone fires whenever they have time – today, tomorrow, next week. You both need an account; the game then belongs to your accounts and continues on any device.

• “Games” shows where it's your turn; a number in the menu counts along.
• If someone does nothing for 14 days (firing or placing), the other can claim the win.
• Invitations nobody accepts expire after 14 days, finished games are deleted 30 days after the end.

For a game against a human you get a link and a token (for example ABCDE-FGHJK). Send one of them wherever you like. Whoever opens it and clicks “Accept the game” sits down opposite you. While you wait, you can already place your fleet.

Anyone who opens the link after that is watching. Spectators see the shots on both seas but no ships until they are sunk – so nobody can cheat with a second browser.

“Resign” (A, asks first) ends the game right away as a loss – during placing too.

After the end you can ask for a rematch. If both want one (the computer always does), it goes on with the same rules, and this time the other fires first.

The server is the referee. Only it knows both fleets, checks every shot and keeps the clock. Your browser only gets what a shot reveals about the other fleet: water, hits, sunk ships. Even with developer tools there's nothing to discover.