Terni Lapilli looks exactly like tic tac toe until you actually start playing it. Then you realise the rules are different in one important way, and that one change makes the whole game feel new again.

Here is everything you need to know to play it.

What you need

A 3x3 grid. That is it. Romans scratched theirs into pavement and stone. You can use a pen and paper, chalk on a driveway, or play it free in your browser if you do not want to draw anything.

Each player needs three pieces of their own. Traditionally these were pebbles of two different colours. A pen and paper version works just as well with X and O, the same as regular tic tac toe.

The two phases of the game

This is the part that catches people off guard. Terni Lapilli has two distinct phases, and most of the strategy lives in the second one.

Phase one: placing

Players take turns placing one piece at a time on any empty square. This goes on until all six pieces (three per player) are on the board. So far this is identical to regular tic tac toe.

If someone manages to get three in a row during the placing phase, the game ends right there, same as usual. But with only three pieces each, this is harder to pull off than it sounds, and most games move straight into phase two without a winner yet.

Phase two: sliding

Once all six pieces are down, the placing phase ends. No new pieces ever enter the game again. From this point on, on your turn you must slide one of your own pieces to an adjacent empty square. Adjacent means up, down, left, right, or diagonally next to where the piece currently sits. You cannot jump over other pieces and you cannot move into a square that is already occupied.

The game continues like this, back and forth, until one player gets three of their own pieces in a row.

This sliding phase is where Terni Lapilli stops resembling tic tac toe at all. The board never fills up. There is no stalemate from running out of space. The game can, in theory, go on for a long time if both players keep blocking each other.

A worked example

Say the placing phase has just ended and the board looks like this:

 X | O | X
 O | _ | _
 _ | O | X

It is X's turn. X cannot place a new piece, only slide an existing one. Looking at the board, the bottom right X is already part of a near-complete diagonal with the top left X. If X slides the top right X down to the empty centre square, X would have pieces at top-left, centre, and bottom-right, completing the diagonal. That is a winning slide.

This is the core skill of Terni Lapilli. You are not just thinking about where to place pieces, you are constantly looking a few slides ahead to see what becomes possible once particular squares open up.

Strategy tips

Control the centre if you can. Just like regular tic tac toe, the centre square is involved in more potential winning lines than any other square. Whoever controls it in the sliding phase has more options on every turn.

Watch what opens up after your opponent moves. Every slide changes which squares are empty, which changes what is possible next turn. A square that was useless two moves ago might suddenly be exactly what you need.

Do not get fixated on one line. Because pieces keep moving, a winning setup can fall apart in a single slide if your opponent moves the right piece out of the way. Keep more than one potential line open if you can.

Force your opponent into bad slides. Sometimes the best move is not advancing your own position but limiting your opponent's options so badly that whatever they slide next helps you.

How it is different from tic tac toe

In regular tic tac toe, the game is solved. With perfect play from both sides it always ends in a draw, every time, no exceptions. Terni Lapilli has not been studied nearly as thoroughly because almost nobody plays it anymore, which means the optimal strategy is far less established. There is genuine room to discover things playing this game that nobody has fully mapped out.

That alone makes it worth trying, even if you have played thousands of games of regular tic tac toe and gotten bored of always drawing.

Where it came from

Terni Lapilli is Latin for "three pebbles." Archaeologists have found the distinctive grid scratched into stone surfaces all across the Roman Empire, including the steps of the Colosseum. The earliest known examples date to around 100 BC, making this one of the oldest strategy games still playable today. You can read the full history of the game if you want the longer story.

Play Terni Lapilli free →

Sources