A domino is a small rectangular wood or plastic block, thumb-sized, with one face bearing an arrangement of dots or spots resembling those on dice (28 such pieces make up a complete set). A domino’s identity-bearing side also has a line or ridge that separates it into two squares, each marked with an identical pattern of the same sort of dots. In this design, the two squares are called pips, and the pips on a domino are either white or black.

Dominoes are stacked on end in long lines and then tipped so that their adjacent edges match. This makes it possible to build a chain of dominoes that can extend for very long lengths, each domino leading to more and more dominoes until the entire line tips over. Very complicated designs can be built in this way, and there are many different games that can be played with them.

Each player in turn places a domino on the table, positioning it so that its pips match those of an existing tile on the ends of its row. The new domino may be placed either to a single, or to a double. In the latter case, a tile must be played such that both of its matching ends are adjacent to another domino already on the table, with the other end of the new domino touching one of the two pips on the adjacent edge of the old domino.

In many domino games, the first play of a piece is made by the person who draws the highest domino from the stock. This is sometimes referred to as the “set,” the “down,” or the “lead.” If there is a tie, it is broken by drawing additional dominoes from the stock. Some games specify that the heaviest domino must be played first, while others allow the player to choose the play, regardless of its weight.

In stories, the domino effect is used to suggest a chain of events that starts with a simple action and then leads to much larger–and occasionally catastrophic–consequences. It is a common trope in thrillers, mysteries, science fiction, and other genres, but it can be used by any writer who wants to evoke a sense of dramatic momentum. The trick is in the timing. Whether the writer is writing off the cuff, or taking time with a careful outline, she has to remember that every scene should move forward toward the story’s goal. If the pace of a scene feels slow, or the reader is not sure what will happen next, they will quickly lose interest in the story. It is not enough for a scene to move the hero closer to victory, but the victory must feel earned, and that requires proper timing.