This is an excerpt from C.E.Forman's 1995 article in "XYZZYnews Issue #4: Game Design at the Drawing Board", which was once available at http://www.xyzzynews.com/xyzzy.4e.html, but which is no longer available there.

Puzzle Structure Chart

This is my own personal favorite design tool, but unfortunately it's also the most difficult for me to describe in words. A puzzle structure chart is much like a hierarchy diagram which lists the necessary puzzles in an adventure game, from beginning to end, and indicates what must be done before each puzzle can be solved. Confused? Maybe it'll become clearer if you look at the example below. This is the complete puzzle structure chart for Infocom's "Enchanter" (and just in case you overlooked the first warning, the following diagram contains some really big-time spoilers): 

                                    START
                                      |
   ___________________________________|__________________________________
  |       |                                                              |
Open    Fill                                                            Get
oven     jug                                                          REZROV
  |       |                                                              |
  |       |                                                              |
 Eat    Drink                                                         REZROV
bread   water                                                          gate
  |       |                                                              |
  |       |                                                              |
  |      /                                                             FROTZ
  |    /                                                              an item
  |  /    _______________________________________________________________|
  |/     |         |         |         |       |           |           |
  |    Open      REZROV    Sleep     Open    NITFOL      Enter       Reach
  |     egg     n. gate   in bed     cell    turtle     Gallery       in
  |      |         |         |         |       |       w/o light    rathole
  |      |         |         |         |       |           |           |
  |       \       Get      Open      Move     Lead         |           |
  |         \    KREBF    bedpost    block   turtle      Move         Get
  |           \    |         |         |      into     portrait      GONDAR
  |             \  |         |         |     Engine        |           |
  |               \|        Get       Get     Room         |           |
  |                |       VAXUM     EXEX      |          Get          |
  |                |         |         |       |         OZMOO         |
  |               Fix        |         |       |           |           |
  |              ZIFMIA      |       EXEX      |           |           |
  |                |         |      turtle     |         Cast          |
  |                |         |         |       |         OZMOO         |
  |              Summon      |         |      /           on           |
  |            adventurer    |         |    /          yourself        |
  |                |        /          |  /                |           |
  |                |      /            |/                  |           |
  |                |    /              |                Survive        |
  |                |  /                |               sacrifice       |
  |                |/              "TURTLE,            in temple       |
  |                |             BRING ME THE              |           |
  |                |                SCROLL"                |           |
  |              VAXUM                 |                 Open          |
  |            adventurer              |               jewelled        |
  |                |                   |                  box          |
  |                |                   |                   |           |
  |              Lead                  |                   |           |
  |            adventurer              |                  Get          |
  |            to guarded              |                MELBOR         |
  |             doorway                |                   |           |
  |                |                   |                   |           |
  |                |                   |                 Cast          |
  |           "ADVENTURER,             |                MELBOR         |
  |            OPEN DOOR"              |                  on           |
  |                |                   |               yourself        |
  |                |                   |                   |           |
  |             Get map                |                   |           |
  |            and pencil              |                 Enter         |
  |                |                   |                 area          |
  |                |                   |                beyond         |
  |             Release                |               junction        |
  |          Unseen Terror             |                   |           |
  |                |                   |                   |           |
  |                |                   |                  /            |
  |            Recapture               |                /              |
  |          Unseen Terror             |              /                |
  |                |                   |            /                  |
  |                |                   |          /                    |
  |               Get                  |        /                      |
  |              GUNCHO                |      /                        |
  |                |                   |    /                          |
  |                |                   |  /                            |
  |                |                   |/                              |
  |                |                   |                               |
  |                |                   |                               |
  |                |                 KULCAD                            |
  |                |                 stairs                            |
  |                |                   |                               |
  |                |                   |                               |
  |                |                 Cast                             /
  |                |                 IZYUK                          /
  |                |                  on                          /
  |                |               yourself                     /
  |                |                   |                      /
  |                |                   |                    /
  |                |                  /                   /
  |                |                /                   /
   \               |              /                   /
     \             |            /                   /
       \           |          /                   /
         \         |        /                   /
           \       |      /                   /
             \     |    /                   /
               \   |  /                   /
                  \|/                   /
                   |                  /
                   |                /
                 Enter            /
                Krill's         /
                 tower        /
                   |        /
                   |      /
                   |    /
                   |  /
                   |/
                   |
                   |
                 GONDAR
                 dragon
                   |
                   |
                 VAXUM
                  (or CLEESH)
                 shape
                   |
                   |
                GUNCHO
                 Krill
                   |
                   |
                  END

Okay, let's start at the top of the chart and work downwards. At the start of "Enchanter," there are really only three significant tasks for the player to accomplish--opening the oven to find the bread, filling the jug with water, and getting the REZROV spell. The three separate paths leading off from "START" reflect this.

Note also that only one of these paths, finding the REZROV spell, really opens the doorway to further adventure. Eating and drinking are merely necessary to keep the player alive long enough to reach the end. So the game starts out almost purely linear, requiring players to find the spell, use it to enter the castle, and use the FROTZ spell to create a light, before anything else can be accomplished. Once the player gets past these first few minor obstacles, however, the scope of the puzzles reaches its widest point. The game is now quite non-linear, with seven distinct paths that players are able to follow.

As an example, let's assume that the player's first task is to open the jewelled egg and obtain the shredded ZIFMIA scroll inside. Let us assume that the player then proceeds to cast the REZROV spell on the castle's rusted north gate in order to gain access to the forest beyond, where the scroll inscribed with the KREBF spell is located. The player now has the KREBF spell and the shredded ZIFMIA scroll itself, both of which are necessary in order for the shredded scroll to be fixed. So the two initially separate paths merge into one, and the KREBF spell can now be used to obtain the ZIFMIA spell, which in turn can be used to summon the wandering adventurer.

In a puzzle structure chart of this nature, when a path splits into two or more separate pathways, this indicates that there is more than one puzzle for the player to solve at that particular point in the game. When two (or more) paths merge into one, however, it indicates that both paths must be completely traversed before the player can continue. You couldn't obtain the ZIFMIA spell without the KREBF spell, nor could you get ZIFMIA if you didn't have the shredded scroll on which it is written in the first place. The puzzles leading up to the acquisition of both items must be solved before the two items can be used together to solve a third puzzle.

And what about optional puzzles, and puzzles with more than one solution? Well, to keep the above example less cluttered, I didn't include the CLEESH spell, since it isn't essential to victory. I did make a note that it could be used in place of VAXUM when confronting the shape in the endgame. The discovery of the CLEESH spell could have been inserted on a separate path branching off of the path in which the player enters the forest and finds the KREBF scroll, and later tied into the endgame.

With a puzzle structure chart, however, neither of these are absolutely necessary. A puzzle structure chart allows the designer to explore a game's linearity, to determine if it's too rigid or too vast and overwhelming. It lets a designer see if changes in the arrangement of the game's puzzles need to be made, and if so, where. Puzzle structure charts also eliminate circular logic in puzzles, that is, puzzle A needing an item that can only be obtained by solving puzzle B, which in turn can only be solved with an item from puzzle A. If such a situation exists, it will reveal itself in a puzzle structure chart.