A Story Limit works to bring the story to a climax and a conclusion. This Limit can be accomplished in either of two ways. Either the characters run out of places to look for the solution or they run out of time to work one out. Running out of options is accomplished by an Optionlock; a deadline is accomplished by a Timelock.
Choosing a Timelock or an Optionlock has a tremendous impact on the nature of the tension the audience will feel as the story progresses toward its climax. A Timelock tends to take a single point of view and slowly fragment it until many things are going on at once. An Optionlock tends to take many pieces of the puzzle and bring them all together at the end.
A Timelock raises tension by dividing attention. An Optionlock raises tension by focusing it. A Timelock increases tension by bringing a single thing closer to being an immediate problem. An Optionlock increases tension by building a single thing that becomes a distinct problem.
Both of these means of limiting the story grow stronger as the story progresses. Optionlocks limit pieces with which to solve the problem and can create a feeling of growing claustrophobia. Timelocks limit the interval during which something can happen and can create a feeling of growing acceleration. Both types of Limits bring the story to a climax.
One cannot look just to the climax, however, to determine if a Timelock or Optionlock is working. A better way to determine which is at work is to look at the nature of the obstacles thrown in the path of the Protagonist and/or Main Character. If the obstacles are primarily delays, a Timelock is in effect; if the obstacles are caused by diversions, an Optionlock is in effect. An author may feel more comfortable building tension through delays or building tension through diversion. Choose the kind of limit most meaningful for you and most appropriate to your story.
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