adding my 2 dollars worth
Just calling a prologue the first chapter does a disservice to the work, if it truly is a prologue. It is important to remember that the prologue is a tool in a writer's box that can be used to promote a different view of the world or to introduce an important trait to the story.
Prefaces are also a tool that can be used, among other things, to glean sympathy, remorse, hate, love, or pure indifference to a character or group of characters.
I used a preface for a book to introduce the POV character and to ascribe several many points about the character so that it wouldn't be an info dump in the book. Written as an introductory dedication, I was able to bring out the character's family, friends, and set the tone of the language for the book. it could have been done over the length of the book, but this got the point across before the important reading took place. I was able to enter the story immediately.
My present WIP involves a prologue. The story is based around November 1963 in Dallas, and parallels the Kennedy assasination. In order to set time, location, and tie the whole thing to Dallas later in the story, I have used the Kennedy murder as a backdop in the story to re-invent the facts to fit my plotline.
The one thing I have gathered from experimenting with this is that any information a reader needs that is in the Preface/Prologue will have to be repeated later.
How this works? We'll see.