Before DH was released, people had theories that the books were symmetrical and were trying to predict what was going to happen next.

i.e. Books 1 & 7, 2 & 6, 3 & 5 have elements in common.

Here are a few that I have quickly brainstormed:

Books 1 & 7 - We find out that Snape is actually the good guy after thinking he is bad, Mr Ollivander and Griphook make appearances, the poem above the door at Gringotts is brought up

Books 2 & 6 - Tom Riddle's diary and the book that belongs to the Half-Blood Prince, the little excursions to cold, dark places to destroy horcruxes (Chamber of Secrets and the Cave), Harry and Draco both visit Borgin and Burkes, a vanishing cabinet is found in Borgin and Burkes

Books 3 & 5 - Learn of the Marauders, the Knight Bus, Sirius Black, there is a hearing (Buckbeak's and Harry's)

I find this a very fitting and interesting theory, and I want to know what others think about it.

You know what? That's really interesting! Lol. The only problem is, I don't see how people could've made accurate predictions on book seven using book one. Because a lot of things happen (big and little) and it would be just too hard to actually find something important enough to link to what might happen in the last book.

But, still, I think this is really interesting! Wonder if that was done on purpose, or accident?

Yeah I think they are intended to be symetrical. Either that or Jo didn't realise til book four because she said that one was pivitol in the series in both the sense that it is the change from a non-coporeal Voldemort to a now corporeal Voldemort.

In terms of structure PS and DH are similar as they are both books with broken up adventures whereas the rest are all part of one linear plot whereas PS and DS zig zag towards the end.
Plus in PS and DH Harry rides on Sirius' motorcycle with Hagrid away from danger to safety. It is revealed at the end that Snape turns out to be saving Harry. Harry visits Gringotts in both the only two times he does it in the series. He also encounters a dragon in PS and DH.
In COS and HBP Harry puts his trust in a book he just finds lying around. He also visits Knockturn Alley in both and sees Draco and Borgin and Burkes in both. Also the students were almost sent home in both.
In POA and OOTP Harry sees Lupin, Sirius, the Knightbus. He also learns about the circumstances of his parents deaths in both. His patronus and the dementors appear in both.
Alot of parallels.

I think PoA and HBP are the most similar out of all of them. Both books are all about revealing back story and hooking us into the next part of the story. PoA ties up loose ends from 1 and 2 and prepares us for the next segment, and HBP prepares us for the finale that was ahead.

So in my head, I always had the structure look like this:

1 is similar to 4
2 to 5
3 to 6
and 7 is a standalone, unlike anything else in the series (since it's not Year 7 at Hogwarts anymore).

In both SS and GoF, Voldemort has a scheme to return back to a body. Harry learns about the wizarding world in the first book, and even in the fourth book there were many references to the fact that Harry still has a lot to learn (Foreign schools, Giants having a stigma, relationships); both books were a lot about self-discovery. Also, both books have a set of dangerous tasks that Harry must get through, with the help of his friends.

CoS and OotP both were similar in tone. In both, Harry was an outcast at school and had to deal with everyone talking behind his back. Everyone thought Harry was something that he wasn't (Slytherin's Heir, a nasty liar). Dumbledore was forced out of school in both, and Harry had to save a friend in both (Ginny, and he thought he was saving Sirius).

And like I said, PoA and HBP seemed like transition books to me. Both were devoid of Voldemort's direct interference. Both were about discovery of the past (in PoA, Harry learned it was Wormtail who sold out Harry's parents. In HBP, Harry learned that it was Snape who originally gave Voldemort the information on the prophesy).

But what this all boils down to is that we can all find patterns between any of the books and come up with a structure. I think Rowling's books were fairly tight and well-thought out from the beginning (unlike say, Star Wars).

That is a very good point you raise, astralpictures. I agree completely.

It's an interesting theory, but a little far-fetched, I think. And I agree with you, astralpictures, that book 7 is unlike any other in the series (no Hogwarts Express, no Hogwarts lessons, etc.) and that there are more similarities between books 1 and 4. In both Snape is suspected to be the villain, but in the end it turns out it's someone else.

That's quite interesting!
I've never picked up on that....