Although tree structures are natural and powerful, they do have significant limitations. Anybody creative can run up against these limits. We’ve already mentioned a couple tricks: links and clones. Here’s another really significant one that takes advantage of the two dimensions of your screen.

Columns are a simple idea; outline headers have a “main” column that behaves like an outline and additional columns to the right that behave like a table, or in some cases like a spreadsheet. More advanced implementations allow columns to have assignable data types (numbers, text, dates, events, and so on) and styles.

Sometimes they can have computed results. Often, they are sortable, which temporarily (or permanently) changes the order of headers, acting as row titles. Clever use of attributes in columns and sort criteria can emulate a search and gather function. InfoDepot, now defunct, seems to have developed this idea, followed by the similarly defunct INcontrol. They had masterful implementations of columns. OmniOutliner is the only OS X outliner that supports this type of column, and its implementation is pretty robust. The screenshot shows OmniOutliner.

Schedule is multi-platform but originated on the Mac. It also has a column feature from its Mac-only days, which is much more mature but not as general purpose.

Often those columns can support pop-up lists of user-defined variables. These usually serve as a way of tagging the header, for instance you may note whether a paragraph is in draft or finished—or whether the priority of an item is high or low. But there are more robust, database-like ways of assigning or extracting keywords and developing indices to help you find an entry. Many users deem this important because files often get huge, especially if content is imported from elsewhere.

NoteBook has a relatively simple tagging capability, and a large number of automatically generated, handy indexes on those tags. Hog Bay Notebook uses a strong open source search engine (but can’t tag).

The example screenshot for this feature is of DEVONthink. Other outliners allow you to assign keywords and categories to notes and sometimes text within notes. DEVONthink assumes that most of your notes will be imported, that there will be huge masses of them, and that the organization of these notes will be mostly extracted rather than applied.