Time for a change in pace.  Narrative Structure, in the sense used in this post, refers to the flow and progression of the story.  Arguably, the narrative structures mentioned here are more prominent in interactive media, but they can just as easily appear in books or movies.

Linear

This is the simplest and most straightforward structure.  You start at the beginning and continue forward until you reach the end.  Strictly speaking, Linear structure tends to be the superstructure of any narrative, but there are rare cases where there is no sense of Linearity.

Branching

This is a slight variation from Linear.  In a Branching narrative, there are side paths, but they return to the core narrative while retaining the status quo.  A filler arc in a television show is a Branch.  A scene in a movie that provides no contribution to the plot is a Branch.  In dungeons in RPGs, Branches manifest as “go the wrong way for more treasure.”  Branches aren’t always bad; they aren’t always detrimental to the narrative.  For example, a Branch can provide characterization of other insights into the narrative or interpretation, without actually pushing the plot forward.  Arguably, Citizen Kane‘s narrative is a reporter trying to figure out what “Rosebud” means.  90% of the movie, then, is Branches.

Multipath

Another variation of linear.  Multipath is like branching, but the branch rejoins the story at a different point than the departure, allowing two routes through the narrative.  This is easier to see in video games, where one path takes you over the mountain, while another takes you through the mountain.  At the end, you’ve arrived at the same point, but you took different routes to get there.  Super Mario Bros. is on the surface Linear, but secret pipes, beanstalks, and warp zones reveal the hidden Multipath aspects.  Super Mario Bros. 2 (Japan)(AKA The Lost Levels)  added reverse warp zones, making the narrative recursive, but still a Linear Multipath.

Forking

Unlike Branching and Multipath, a Forking structure never returns to the source.  Most Choose Your Own Adventure books used a Forking structure, with the occasional instance of Multipath.  The movie Clue was presented in theatres in a forking structure, with three different endings, randomly chosen.  The DVD version presents a choice at the beginning, between the theatrical random ending and the VHS all three endings.  Therefore, the DVD version actually has two Forks – one at the beginning, and one at the end of the former path.

Network

A Network is, in its purest sense, at the opposite end of the spectrum from the Linear structure.  In a Networked narrative, there are several Nodes, or points of interest, and paths between the Nodes, connecting many – but not necessarily all – to each other.  There are few examples of a pure Network narrative, but, in fairness, there are few pure Linear narratives as well.  Memento is probably an effective example of a Network used in a non-interactive medium – there are several points of interest throughout the presentation, but they are not presented in a Linear manner.

Gated Network

A Gated Network is essentially a Network with a mild Linear superstructure.  There is something necessary to progress the plot forward, but there is no narrative compulsion to achieve that goal at any specific point.  The Legend of Zelda is a simple Gated Network – you have freedom of movement among nodes, but need some items to bypass obstacles (soft gates), and need to complete narrative goals to reach the final dungeon (hard gate).

Putting it Together

Any specific example is likely to use many of these structures in a nested format.  The Megaman series is a gated network with a hard gate (defeat all the robot masters) leading to a linear second half.  In the earlier games, the Network is a Linear Network, as each node can only be visited (completed) once.  Each node within the Network (as well as the post-Network levels) is itself a Linear narrative, generally with Branches and the occasional Multipath.  Later games even experiemented with Forks in the sub-nodes.  Mass Effect 2 presents Linear interpersonal narratives with Forks.  Each conversation is a combination of Linear Multipath and Linear Fork, as well.  This is all encapsulated in a Gated Network which is seeded with a few linear nodes.  And event hat comes off as an oversimplification.

Flags and Variables

Flags and Variables are not inherent to narrative structure, and largely only present in video games from a technical standpoint, but they are necessary in understanding and dissecting a narrative structure.  Branches, Forks, and Multipaths are occasionally presented as a direct choice, but they are often the result of flags and variables set by previous choices.  Flags are a binary value – it is true or it is false.  When an event triggers a flag, the flag is set to “true” or “false” or, in some circumstances, is toggled (i.e. switched from true to false or vice-versa).  Flags occasionally appeared in Choose Your Own Adventure books that included Branches or Multipath – “If you have the MacGuffin, turn to page 20.  If you do not have the MacGuffin, turn to page 127.”  Variables work in roughly the same way as Flags (i.e. as logic gates), but are incremental.  When an event triggers a variable, a number is added to or subtracted from the stored variable.  In a Choose Your Own Adventure book, they would be presented as “If you have five or more vials of phlebotonium, turn to page 63.  If you have fewer than four vials of phlebotonium, turn to page 16.  If you have four vials of phlebotonium, turn to page 140.”  Returning to the Mass Effect 2 example, certain conversation options require a minimum variable (generally Paragon or Renegade value).  After the loyalty arc for any potential romantic partners, the “Interested in Romance” flag for that individual is set to “true,” and remains true until the wrong thing is said.  When the “Interested in Romance” flag is false, the characters have much less to say, though they may have new dialog due to the “loyalty” flag being true.