A prologue is usually assumed to be a part of the story, or something relating to the story, that happened some time before the story began. Though this is not always an event relating to the story, people have also used it for short mythological or informational pieces that relate to the story or another character’s perspective on something relating to the story. Essentially though a prologue must be linked to the main story in some way and not entirely separate from it. For example, as I mentioned in my previous article prologues are commonly used in crime fiction to show a crime being committed before we are introduced to the detective.
Someone once also tried to tell me that a story with a prologue must have an epilogue at the end like bookends on a shelf. Obviously forgetting that people often only use on bookend and let the wall/side of the bookcase hold up the other end. So you don’t have to have an epilogue just because you have a prologue, trying to stick to a rule like that could lead to adding unnecessary chapters or information to a story simply to satisfy a non-existent rule.
The first thing to consider when writing a prologue is whether you need the scene or whether you story works better without one. If you’re thinking of writing a prologue then the try the story with and without it to see how it works. It’s also a good idea to check out trends in your chosen genre, I’m not saying you have to stick to what other writers are doing simply that might give you ideas for your prologue if you’re struggling. Reading books from other genres would also help when you’re trying to decide whether or not to include a prologue because it would allow you to get a clearer idea of the different ways writers have used the prologue.
On unusual example I can think of is using a prologue that is an excerpt from later in the story to hook a reader. However, in my experience this isn’t always an effective tool for hooking a browsing reader. The problem with constructing a prologue as a hook is that due to its reputation for generally being different from the rest of the story, such as have a different point of view, the reader is more likely to skip the prologue and look at the first chapter as their sample. So I would suggest, based purely on my experience, that it would be better to use the excerpt from later in the book as a narrative ploy beyond trying to use it as a hook. What this narrate trick might be I couldn’t say because I can’t think of one, but if you can give it ago.
However you use your prologue it needs to be relevant information because readers know that prologues tend to have information they might need later in the story so it sticks in their mind. If it doesn’t then the reader will likely be disappointed like someone trying to assemble a jigsaw puzzle only to discover one of the pieces is from a different puzzle.
For more writing advice see my Advice Page and for more on narrative structure see Finding Your Voice.