Descriptive book
From MYSTlore
Descriptive Books are used to establish links to Ages by the D'ni.
A description of the Age is written into the book using a particular form of D'ni (whether or not another language can be used is currently unknown). The link to a specific Age would be established when the book was first used. All described features would be present but others that had not been described would also be found as the link would attach to a matching world at random.
[edit] Making changes to descriptive books
Even though the book does not create the Age it links to, it can be used to make changes to it. This practice is extremely dangerous because it can introduce instabilities into a previously stable Age. For example, it's implied that Riven was more stable before Gehn made contradictory changes in the descriptive book.
Although changes can be successfully made by a sufficiently skilled writer (for example the Linking Chambers in Spire and Haven) D'ni society outlawed the practice as too dangerous.
Crossing out words in the descriptive book results in even more catastrophic effects. Instead of forcing changes into the Age, the book shifts its link to a different Age that now more closely matches the description in the book, which means that the link to the original Age will be lost forever. This is what happened to Gehn's 37th Age.
Big or uncareful changes to the descriptive book can also make the book stop working altogether. It seems however that the damage can be undone. As shown in Myst, one way to temporarily disable a book is to remove one or a few pages from the descriptive book; when they are put back in the right place, the link will work again (note however that Myst's presentation of linking mechanics are highly laden with AL; this might be another example of that).
[edit] Other restrictions
Another law that restricted writing Descriptive Books was the banning of writing man-made objects into an Age as experiments in this practice resulted in highly unpredictable results.
D'ni society also outlawed writing links to Ages that defied the laws of physics. This is most likely because such Ages could be harmful to anyone who entered them. Despite this restriction it was not always backed up with law.
