- The Fixed Timeline; if you go back, you have already been back. Any past event that you try to prevent will end up being caused, but you can go back to a MacGuffin needed to solve a future or a present problem
- The Dynamic Timeline; you can go back alone your own timeline, but you may cause a paradox by doing anything that will cause you to never go back in time. It should be noted that this is the most unrealistic version of the timeline, as going back in time already makes a different timeline, and thus paradoxes in this timeline are actually caused by anything you can prove will cause you not to go back.
- The Multiverse Timeline; if you "go back in time", you are actually traveling to an alternate universe, identical to yours up to the point you traveled to. Once in this universe, you cannot return to your own. You simply disappear from your own timeline at one point, and appear in this timeline at one point.
So You Want To / Write a Time-Travel Story
Step One: Establish the rules of time travel in your universe. Every story has different rules - Doctor Who, Harry Potter, Artemis Fowl. It's important to understand what the rules and limitations are in your universe. There's three main ways the timeline can work: