A story in which one small change has a ripple effect, resulting in massive changes.
The trope's name derives from a proverb-turned-poem which traces its origins as far back as the 14th century (making this Older Than Print). Probably the most well-known version is the one which appeared in Benjamin Franklin's Poor Richard's Almanac:
For want of a nail, the shoe was lost;
For want of a shoe, the horse was lost;
For want of a horse, the rider was lost;
For want of a rider, the message was lost;
For want of the message, the battle was lost;
For want of a battle, the kingdom was lost,
And all for the want of a horseshoe nail.
A Deal with the Devil will often have the "nail" as a price, something of seemingly small consequence that is in fact huge. In a series of Disaster Dominoes, the first domino often falls when the "nail" that props it up suddenly goes missing. Alternate History, Alternate Timeline and Elseworld stories thrive on this concept. When applied to Time Travel stories, it is usually referred to as the "butterfly effect".
A Sub-Trope of What If?, and related to Unwitting Instigator of Doom, Small Role, Big Impact, Dice Roll Death, Set Right What Once Went Wrong and Make Wrong What Once Went Right. A Super-Trope of Close-Enough Timeline and Big First Choice (when the player decides about the nail). If the originals meet their alternates, may result in Other Me Annoys Me or Future Me Scares Me. The It's a Wonderful Plot is a Sub-Trope. See Mirror Character. Often overlaps with In Spite of a Nail, because many stories wouldn't be that interesting if everything was different. Compare It Began with a Twist of Fate and Contrived Coincidence.
No Real Life Examples, Please!
