Our first distinction is between “What You See Is What You Get” (WYSIWYG) documentation programs and markup-centered tools. Most desktop-publishing programs and word processors are in the former category; they have GUIs in which what one types is inserted directly into an on-screen presentation of the document intended to resemble the final printed version as closely as possible. In a markup-centered system, by contrast, the master document is normally flat text containing explicit, visible control tags and not at all resembling the intended output. The marked-up source can be modified with an ordinary text editor, but has to be fed to a formatter program to produce rendered output for printing or display.

The visual-interface, WYSIWYG style was too expensive for early computer hardware, and remained rare until the advent of the Macintosh personal computer in 1984. It is completely dominant on non-Unix operating systems today, Native Unix document tools, on the other hand, are almost all markup-centered. The Unix troff(1) of 1971 was a markup formatter, and is probably the oldest such program still in use.

Markup-centered tools still have a role because actual implementations of WYSIWYG tend to be broken in various ways — some superficial, some deep. WYSIWYG document processors have the general problem with GUIs that we discussed in Chapter═11; the fact that you can visually manipulate anything tends to mean you must visually manipulate everything. That would remain a problem even if the WYSIWIG correspondence between screen and printer output were perfect — but it almost never is.

In truth, WYSIWYG document processors aren't exactly WYSIWIG. Most have interfaces that obscure the differences between screen presentation and printer output without actually eliminating them. Thus they violate the Rule of Least Surprise: the visual aspect of the interface encourages you to use the program like a typewriter even though it is not, and your input will occasionally produce an unexpected and undesired result.

In further truth, WYSIWIG systems actually rely on markup codes but expend great effort on keeping them invisible in normal use. Thus they break the Rule of Transparency: you can't see all of the markup, so it is difficult to fix documents that break because of misplaced markup codes.

Despite its problems, WYSIWYG document processors can be very nice if what you want is to slide a picture three ems to the right on the cover of a four-page brochure. But they tend to be constricting any time you need to make a global change to the layout of a 300-page manuscript. WYSIWYG users faced with that kind of challenge must give it up or suffer the death of a thousand mouse clicks; in situations like that, there is really no substitute for being able to edit explicit markup, and Unix's markup-centered document tools offer better solutions.

Today, in a world influenced by the example of the Web and XML, it has become common to make a distinction between presentation and structural markup in documents — the former being instructions about how a document should look, the latter being instructions about how it's organized and what it means. This distinction wasn't clearly understood or followed through in early Unix tools, but it's important for understanding the design pressures that led to today's descendants of them.

Presentation-level markup carries all the formatting information (e.g., about desired whitespace layout and font changes) in the document itself. In a structural-markup system, the document has to be combined with a stylesheet that tells the formatter how to translate the structure markup in the document to a physical layout. Both kinds of markup ultimately control the physical appearance of a printed or browsed document, but structural markup does it through one more level of indirection that turns out to be necessary if you want to produce good results for both printing and the Web.

Most markup-centered documentation systems support a macro facility. Macros are user-defined commands that are expanded by text substitution into sequences of built-in markup requests. Usually, these macros add structural features (like the ability to declare section headings) to the markup language.

Finally, we note that there are significant differences between the sorts of things composers want to do with small documents (business and personal letters, brochures, newsletters) and the things they want to do with large ones (books, long articles, technical papers, and manuals). Large documents tend to have more structure, to be pieced together from parts that may have to be changed separately, and to need automatically-generated features like tables of contents; these are both traits that favor markup-centered tools.