This is a little simpler than Aristotle's idea, but when I'm in the earlier stages of the project, I tend to consider the mid-point to be when the protag. encounters an idea, piece of information or a plot obstacle that spins the story off in a new direction. It doesn't have to necessarily be hugely dramatic, but it throws a wrench in the protag's progress (or maybe presents them with an alternate goal).

that being said, the second act is all about wrench's in the protag's progress. But this is a big wrench I'm talking about here.

I know this isn't always the case, but a lot of mid-points seem to wrap up the problem introduced by the Inciting Incident and introduce a Second Problem to see the film through to the end. For example, in Raiders, they find the Ark at the midpoint - problem solved. Oh wait! The Nazi's have stolen it, they'll have to get it back! Back to the Future's midpoint: they'll use lightning to get Marty back to 1985, problem solved! Just stay here and don't talk to anyone... oh wait! You have to get your parents together first! Without that second story problem, introduced at the midpoint, Marty would just have stayed at Doc's all week in hiding. The second story problem gave us the rest of the film.

Thanks for this, Scott, and als Zaike and MagicWizard for your thoughts. Yes, it was the "classic mid-point", the Act II transition that I was referring to. Partly asking because I am having trouble identifying what my mid=point is in my current work-in-progress. It is kind of a double turning point, something very wonderful happens but then something terrible happens. As far as my whole story structure goes, this pair of events is very significant - but I didn't know whether to identify the good thing as the midpoint, or the bad thing, or the pair of events together. And also, they occur closer to 2/3 the way through, not halfway through - so I have been wondering if that is a problem. And then I have been wondering if I should be thinking that all out so hard, the exact details of the structure, or if I should just go with the natural flow of the story. But thinking of it the way you have outlined, Scott, helps hugely - because that event (especially the first happy part of it) is definitely part of the journey for my characters (dual protagonists) - they have finally let go of their old way and come to a new way. So that helps me a lot, to think if it that way. And thanks, also, MagicWizard, for those examples of second story problems following the midpoint. That's interesting to see that slightly different structure, with those examples. So anothe midpoint issue I am having is from looking at TheScriptLab's outline of 8-sequence stucture (where the midpoint is the end of Sequence 4): http://thescriptlab.com/screenwriting/structure/the-sequence/45-the-eight-sequences So according to their outline, if the ending of the story is positive, the midpoint should also be positive; if it is a tragedy, then the midpoint should also be a low. I am wondering if anyone has thoughts on that idea - whether you agree with it, and whether you can come up with examples that either show that parallel structure working very well (I'm especially interested in the negative version, as my work is a dual-protagonist tragedy along the lines of Thelma and Louise or Romeo and Juliet). Thanks everyone!

I really like Scott's answer here--it's a great summary of 3-act structure. However, there is another way of looking at it; Robert McKee sees the mid-point (a strong reversal in the middle of a story), as an indication that the writer is using a 4-act and not a 3-act structure. I find that 3-act structure has a bit of a tyranny over screenwriters these days and I like how, in his book STORY, McKee examines the possibilities of other structures. Not every story lends itself to 3 acts, although that is by far the most conventional and commercially accepted form.

LN, the way I approach a basic approach to narrative structure (as it relates to screenplays) is both 3 and 4. There is the External World [Plotline] which falls into the traditional arc of three acts (Beginning, Middle, End, hard to get away from the innate reality of that per storytelling). Then there is the Internal World [Themeline], essentially the emotional plot of the screenplay. And there the most common narrative archetype is metamorphosis which I look at as being comprised of four movements: Disunity, Deconstruction, Reconstruction, Unity. [Some Protagonists don't change, some change others, some go through negative transformation, but generally for mainstream movies, they go through a positive metamorphosis.] These movement more or less line up with three act structure this way: Act I - Disunity Act II (First Half) - Deconstruction Act II (Second Half) - Reconstruction Act III - Unity I'm not saying that happens in all stories. I'm not saying it has to be that way. What I am saying is this is a paradigm that works if you are trying to write a commercially viable movie. Furthermore I've not seen anything else around that combines the interplay / connectivity of the Internal and External World. In my view, the most interesting lens through which to look at story is psychological journey, and this approach above is a helpful take on that. Then of course, there are the five primary character archetypes that slot right into this...