What story can you have without a beginning?
None, of course.
But the beginning of your story and where the reader starts reading aren't always the same thing, which is why this element has a different name: the Introduction.
But that begs the question: what's the difference between the introduction and the beginning?
When an author describes the main character as he's lying in bed, telling us that he's not an ordinary boy, that he's on summer holiday with his family, and that he used to live in the cupboard under the stairs, this is an introduction to the world and the characters, NOT the beginning of the action.
Now, when an author describes how a book transports three orphans mysteriously into the past, where they learn that magic is real, THAT is the beginning of the action - and it might not take place on the first page of the book.
The Introduction is the portion of the story that you read before the story begins, often giving you an idea of what's normal for the characters and world you're about to dive into. But here's the thing: the Introduction isn't necessary.
Take something like Sleeping Beauty, for example. The story starts with an evil fairy laying a curse on the princess. There is little or no introduction into who the king and queen are, where the fairy came from, or why the princess is so special that she has not one but four fairies at her christening.
So, if the introduction isn't necessary - if you can leave it off and it doesn't change anything - why include it at all? Simply put; when the audience is introduced to the characters and their world before the inciting incident (that is, before the adventure starts) then the audience has the opportunity to feel the impact of the change that sends your protagonist off into the Unknown.
For a good example of lack of introduction done well, take a look at Shakespeare's Twelfth Night sometime. The inciting incident that kicks the story off (the shipwreck) takes place before the play even starts.
At the other end of that spectrum, a book like Dragonflight by Anne McCaffrey spends the first several chapters setting up the world and the characters before the adventure begins.
The Introduction sets the baseline "normal" for your world and characters, allowing readers to feel the impact of the change that kicks off the adventure. It can be left off without changing the story, but the presence or absence of an Introduction will change the way the story feels.
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