The 1e Dungeoneer's Survival Guide divides campaign types into Linear (railroad, in the non-pejorative sense), Open (ie sandbox), and Matrix:
The Matrix Campaign
The matrix campaign allows the DM to create a detailed story
with a developed plot, while still allowing the players to choose
where they go and how they deal with their challenges.
The key to a matrix campaign design is to create a goal or
series of goals that the PCs are motivated to accomplish. The
matrix campaign is a very effective style of design for creating an
epic or a string of connecting stories. In many ways it represents
a compromise between the linear and open campaign styles of
The PCs in a matrix campaign start out at a location chosen
either by you or by the players. You then provide them with a short
exposition and some clues that open a number of different
options. For example, the characters may discover that a sect of
evil clerics has been gradually gaining control of the land. They
may witness peasants being arrested by the clerics, or see
places of worship for good-aligned deities suffer raids and vandalism.
The next move is up to the players, with a little bit of guidance
from you. Perhaps one PC remembers seeing a temple to the
north, emblazoned with the evil sect’s sign. To the east, recalls
another, is a great temple of good where the PCs could go for
information. Meanwhile, a group of peasants are hauled off into
slavery to the south, and one begs the characters for aid. All the
time, you know that the headquarters of the sect lies to the west,
but you bide yourtime in revealing this information to the players.
With this technique, the players become involved in the story,
yet are not railroaded into a specific task chosen by the DM. You
are prepared for a few courses of PC action: a raid on the temple
to the north, an interview with the sage to the east, or a prisoner
rescue mission to the south.
Depending on the length of the story you wish to create, you
might have clues in each of the three locations point directly
toward the heart of the problem (the evil sect’s headquarters to
the west), or you might continue to expand the matrix. The players
who rescue the prisoners might lead them as an army to
reclaim their homeland. If they visit the good temple, perhaps
they are sent on a quest to gain some item of great power from
the evil temple to the north. If that evil temple was their original
goal, they might gain the magical item on their own initiative.
The matrix can continue to expand for as long as you wish. All
of the different branches eventually steer the PCs toward the
headquarters of the sect, where the climax of the story takes
place. If you wish to move the plot along more quickly, you can
schedule events that occur wherever the PCs are. Events can be
encounters with significant NPCs, visions and dreams, social or
political changes, or quick scenes designed to show the PCs a
pertinent fact.
A matrix campaign must eventually meet a border, beyond
which the PCs are discouraged from passing. Borders can be
designed as either soft or hard.
A soft border is one that turns the PCs back into the story
through their own motivations. Players who resist any pursuit of
the evil sect, for example, might encounter a group of hapless
waifs whose parents are held prisoner. The children appeal to the
players’ sense of decency. In cases where this appeal is fruitless
(many DMs can predict this ahead of time), a mysterious stranger
might offer a reward of valuable gems for evidence of the sect’s
destruction. Judge your players carefully to decide what type of
persuasion motivates them best.
Players who fail to yield to any kind of motivation can be
allowed to occupy the story’s setting for as long as they like, even
if they don’t take part in it. If they attempt to leave the area, however,
they must encounter a hard boundary. This can be a physical
obstacle, such as a high cliff, stormy sea, or trackless desert.
Alternatively, it might be something like a huge army camped in
the PCs path, with a continually increasing series of encounters
with more and more troops. Quest or geas spells serve as hard
borders, but should be used only as a last resort.
The story matrix should contain several ways for the PCs to
approach the final encounter, and several means of dealing with
the challenge created there. Not all of these means must create
the same likelihood of success, but the PCs should not be
doomed to failure in the final encounter simply because they
made a wrong choice at some point in the adventure.
You should also prepare for the possibility that your players
may not have gained enough information about your story line to
figure out the next course of action. Despite a DM’s thorough and
detailed descriptions, an amazing number of players seem to
have no idea of what’s going on. If this happens in your campaign,
you need to gently but firmly show them the path.
Non-player characters can be particularly useful at such times.
Perhaps a henchman speaks up as all the PCs stand around
scratching their heads, and indicates your preferred course of
action. Or the party might encounter someone very wise, such as
an old sage, magic-user, or hermit. This NPC could provide
details on the next step of the adventure in language so plain that
all players should be able to understand it.
Foreshadowing can be effectively employed to show characters
whether or not they are on the right track. Ill omens and portents
of great danger do not always discourage players-after all,
these are some of the ingredients of good adventure-but they
can be presented in such a way that the PCs are able to figure out
whether they are going the right way or not.