Terni Lapilli (Latin: Three Stones) is a game for two players. It is similar to modern Tic-tac-toe, but is of ancient origin. Each player has 3 tokens that they need to place on a board that haaachis 9 spots, in a 3x3 grid. The first player to get his 3 tokens lined up in a straight line (vertically, horizontally or diagonally) wins the game.
The game starts with an empty board, and each player takes turns placing one of their tokens in an empty spot on the board. When both players have placed all three of their pieces, in each subsequent move, they will move a single token of theirs to an adjacent empty square. In order to move between two adjacent squares, there has to be a path connecting them, depicted by a straight line. If both players are skillful, the game can continue indefinitely without anyone being the victor.
The most common variant (Terni Lapilli), played in ancient rome, allows pieces to be moved diagonally, and the 3 in a row to be achieved diagnonally. This variant is also popular in Ghana, and is called Achi there. Another variant that was popular in England is called Nine Holes. Here, the diagonal moves are disallowed, and you win when your 3 pieces are in a vertical or horizontal straight line (diagonals don't count).
Below is an example game. It is the turn of the player with the X token, and he will lose regardless of which move he picks, because the in the next move, the player with O token will move the bottom left token to middle left position, thereby getting three in a row in the middle horizontal row.
When you create a game, you can chose to play either against another player, or against the computer. The difficulty level of the computer player is changed by clicking the COMPUTER button. By default, the Computer players level is NORMAL, which is of average difficulty. If you want an easier game, chose NOVICE, or for a tougher challenge, chose EXPERT.
You place your token on the board by clicking/touching an empty square. Once all 3 pieces are placed, you touch on one of your pieces and then touch an empty square to move your piece to the empty spot. The empty spot must be adjacent, and connected by a straight line to be valid.
If a player wince, or once each player has played 15 moves, the rematch (↺) button appears to start a new gane. The player who had the O pieces now gets the X pieces, and will play first.