Despite the apparent complexity of TeX's job, it uses a very simple metaphor: all typographic elements are boxes. The simplest boxes, individual characters have a set shape defined by the font they come from. There are three parameters that define a box: width, height, and depth. The distinction between height and depth is a bit subtle. When a row of characters is typeset, every character rests on an imaginary line called the baseline. Some characters, like the lowercase “g,” descend below the baseline. The distance from the baseline to the top of a box is its height; the distance from the baseline to the bottom is its depth.

Figure  Figure 1.1 shows the character boxes formed by the Computer Modern Roman letters “g” and “h.” The x-y distance of each box is its height and the y-z distance is its depth. The reference point of the box, marked with an r, is on the leftmost edge of the box where the height and depth meet. Characters that have no descenders (no elements that go below the baseline), have a depth of zero. TeX uses the character box metrics, but font designers are free to allow glyphs to extend outside the box (for example, at the top of the “g”).

Figure 1.1. The Letters “g” and “h” inside their boxes.

The following paragraph demonstrates how TeX uses the metrics from the physical dimensions of each character to build word, line, and paragraph boxes.

TeX “glues” character boxes together to form words. When boxes are joined, they are always joined so their reference points are horizontally aligned as shown in Example 1.1.^[3] Character-boxes (like this) are joined to form words, word-boxes (like this) are joined to form lines, and line-boxes form paragraphs. TeX accomplishes the task of forming a justified paragraph by allowing the glue between words to stretch and shrink a little bit and by occasionally breaking the glue between characters to insert a hyphen. Although the rules are slightly different, TeX builds a page out of vertical boxes (paragraphs, figures, etc.) in an analogous manner.

This is a very generalized overview. In reality, a lot of subtlety is required to capture all of the nuances of typographical appearance.

A control sequence is a special “word” that you put in your document. These extra words are instructions for TeX, and they do not usually appear in your typeset document. Example  Example 1.1 shows a contrived example of a TeX document that uses several control sequences.

\def\ora{O'Reilly \& Associates}
\font\orafont=grlg10
\parskip=\baselineskip
\parindent=0pt
\pageno=5
This book is published by \ora in
the \textit{Nutshell} series.
\bye

In most macro packages, a control sequence is a backslash followed by a sequence of letters.^[4] TeX is case-sensitive, so the control sequence \large is different from \Large (these control sequences switch to large and very large fonts in the LaTeX macro package). Control sequences end with the first non-letter, even if it isn't a space. For example, \parskip0pt is the control sequence \parskip followed by 0pt. This control sequence tells TeX to insert zero points of extra space between paragraphs.

Unless instructed otherwise (with control sequences), TeX builds rectangular paragraphs out of lines of words. Changing fonts, building tables, and typesetting mathematical equations are examples of situations in your document where TeX needs extra information.

The number of control sequences used in a TeX document may seem overwhelming at first. Luckily, every control sequence falls into one of several categories:

The number and kind of control sequences available depends upon the macro package that you are using. (Macro packages are discussed fully in Chapter Chapter 4, Chapter 4.) For the rest of this chapter, the default settings of Plain TeX are assumed.^[8] There are other macro packages, like LaTeX, Lollipop, and TeXinfo, which have different default values.

In addition to control sequences, TeX reserves several characters for special purposes. Most of them do not occur very frequently in ordinary text, but you must be aware of them because there will be very surprising consequences if you use them incorrectly.

Table  Table 1.1 shows all of the special characters in Plain TeX.^[9] Most of these characters are special in other macro packages as well. Font-specific characters are not reserved by TeX, but they don't produce the results you would expect when typeset in Computer Modern because of the way TeX expects fonts to be laid out. Fonts are discussed in detail in Chapter Chapter 5, Chapter 5.

It is best to avoid these characters until you are familiar with TeX. If you need to typeset one of these characters, Table 1.2 shows what to put in your document. You should also avoid characters outside the standard printable ASCII character set (characters with ASCII values below 32 and above 126). TeX can be configured to accept characters outside the printable ASCII range, to support non-English languages, for example, but it is not configured to do so “out of the box.” Chapter  Chapter 7, Chapter 7, discusses the issues of typesetting in different languages.

\def\bs{\char92\relax}

The braces “{” and “}” are a very special case. TeX uses curly braces to delimit arguments and make changes (like switching fonts) that are local to a small section of the document. These are called grouping characters in TeX jargon. For example, to typeset a single word in boldface, you put {\bf word} into your input file. The \bf control sequence switches to boldface type, and the curly braces localize the effect to the single word word. As a result, it is very important that you avoid braces (except when you use them as delimiters) and that you carefully match all opening and closing braces. One of the most common errors in TeX is to forget a closing brace.

One last special character is the blank space. For the most part, TeX doesn't care how you space your lines of text. Any space that occurs is simply a word break to TeX, and inserting multiple spaces doesn't influence how TeX typesets the line. TeX also considers the end of a line an implicit space. If you are trying to control the layout of your input text and want to break a line without introducing a space, place a comment character (% in most macro packages) at the very end of the line. If the last character of a line is the comment character, TeX ignores the line break and all the leading spaces on the following line. This allows you to use indentation to make your input file more readable. \goodbreak

For example, the following lines in your input file:

“This                      is some ex
      ample text.”

and this line:

“This is some example text.”

both produce:

in your typeset document.

Desktop publishing systems like Ventura Publisher and Aldus PageMaker are noted for their ability to incorporate multiple fonts and graphics into a document. As word processors become more sophisticated, the line between word processing and desktop publishing is becoming blurry.

This book shows you many ways that TeX can provide access to the same sophisticated features. TeX can incorporate pictures and figures in a number of ways (just take a look at the way I've wrapped text around this kiwi),^[11] and TeX can use almost any font that another program can use---it can certainly use all of the popular types of fonts. Like typical word processors, desktop publishing programs force you to use a single application to create your entire document, and they lack the flexibility required to combine just the pieces that you want. All of the advantages of text formatting over word processing also apply to desktop publishing programs. I'll grant, however, that WYSIWYG environments are easier for first-time users. But that doesn't make them better, it just makes them more popular.

troff is the “other” text formatting system. If you've ever tried to read a unix reference page without formatting it first, you've seen troff. For a long time it was distributed as part of all unix systems. Now it is more likely an extra-cost option. The Free Software Foundation's groff processor is a free, troff-compatible system.

On the surface, it is easier to compare TeX and troff than to compare TeX to the other document preparation systems described in this chapter. In reality, the differences are subtle: TeX and troff have the same general paradigm; they are equally powerful to a large extent, and both have advantages and disadvantages.

troff is similar to TeX in many ways. Like TeX, troff processes a plain text file and produces a typeset document. TeX and troff differ in the way that formatting information is inserted into the text. TeX uses control sequences, where troff uses a mixture of control sequences^[12] and “dot” commands (lines of text that begin with a period and contain typesetting commands).

Although I am inclined to say that troff documents are far more cryptic than TeX documents, I am certain that there are plenty of troff users who would disagree (strongly).

Objectively, TeX handles mathematical typesetting far better than troff and probably has better support for multilingual documents. The nroff processor, which produces plain text output from a troff document, at one time provided a strong argument in favor of troff for typesetting documents required in both typeset and plain text formats. However, the TeXinfo macro package for TeX has largely defeated that argument. In troff's defense, TeXinfo is very, very different from other TeX macro packages, so it really is necessary to plan ahead and learn a very different set of macros to typeset both plain text and typeset documents with TeXinfo. Chapter Chapter 10, Chapter 10, discusses this issue further.

In my experience, there is more free support for TeX than troff. TeX is supported by a large community of users actively producing new, useful document-preparation formats, styles, and tools. In addition, TeX is more widely available than troff: a TeX port exists for almost every practical computer system, whereas troff is still mostly confined to unix systems (although the Free Software Foundation's groff package has been ported to similar systems like MS-DOS, Windows NT, and OS/2).

The following fragments show a side-by-side comparison of TeX commands, on the left, and troff commands, on the right:

\begin{figure}                    .(z
 \begin{center}                   .hl
   \hrule                         Text to be floated.
   \vspace{8pt}                   .sp
   Text to be floated.            .ce
   \hrule                         .hl
   \caption{Example figure...}    Figure \*[fig]: Example figure...
   \vspace{8pt}                   .)z
 \end{center}
\end{figure}

Both examples produce a floating figure that looks like this:

The Standard Generalized Markup Language (SGML) is a document description language. SGML aims to separate the content of a document from its presentation. In other words, SGML identifies the features of a document (chapter headings, paragraphs, etc.) without specifying how they are to be presented.

This means that all SGML documents must interact with a document formatter of some sort. Many people are finding that TeX is a natural choice when selecting a document formatter for their SGML environment. In fact, LaTeX already provides many SGML-like commands because it was designed to separate markup from presentation. One of the specific goals of an effort (currently underway) to develop a new version of LaTeX is to make SGML and LaTeX work together easily, cleanly, and efficiently. For more information about the goals of this project and information about what you can do to help, please read The LaTeX3 Project [l3:project].

This section briefly describes what you need to know about how TeX processes a simple document (that is, one that does not contain complex document elements like a table of contents, indexes, bibliographies, etc.). Figure Figure 1.3 shows how the standard TeX tools fit together at the most basic level.

Figure 1.3. A high-level view of TeX

Figure  Figure 1.4 expands on Figure Figure 1.3, showing additional tools and files that you'll often need to use.

Figure 1.4. High-level view of TeX including more detail

This section briefly describes how TeX processes a more complex document (that is, one that includes elements like a table of contents, indexes, bibliographies, etc.).

Many TeX formats implement sophisticated cross-referencing schemes. Cross references may sound rather esoteric, but they occur frequently. Tables of contents, figure and table numbers, indexes, and bibliographic references are all flavors of cross referencing.

Cross references make your document more complex because they require more information than is immediately available when TeX initially processes your document. For example, if you refer to a figure which occurs later in the document, TeX has no way of knowing what figure number to insert into the text at the point of the reference. These are called forward references.

TeX macro packages that support cross referencing overcome the difficulty of forward references by requiring you to process your document more than once. Each time your document is processed, the necessary reference information is stored into a separate file. If that file exists when you process your document, the information saved last time is loaded so that it is available this time. The practical implication of this functionality is that documents with cross references frequently have to be processed twice. Occasionally, you may have to process a document three times. This occurs when the inserted reference causes TeX to format a paragraph differently, which in turn causes TeX to change a page break.^[16] Because most changes are incremental while revising a document, this is normally only an issue the first time you process a document.

The following sections describe the LaTeX methods for constructing a table of contents, figure references, an index, and a bibliography. LaTeX is used in this example because it is a very common macro package and is typical of the way macro packages provide these features. Similar mechanisms exist in most formats, except Plain TeX.

Figure  Figure 1.5 shows the relationships between many of the components described in the following sections. LaTeX creates several sorts of auxiliary files depending on the kind of cross references required by your document and the style files you use. These auxiliary files may be modified (and others may be created) by other sorts of post-processing programs (like MakeIndex for constructing indexes or BibTeX for constructing bibliographies). LaTeX uses these auxiliary files, if they exist, to update your document when it is processed again.

Figure 1.5. TeXing a More Complex Document

A table of contents is the simplest form of cross reference. In LaTeX, you request a table of contents by inserting the \tableofcontents command wherever you want it to appear in your document. If you request the table of contents at the end of your document rather than the beginning, your document can be printed with only one pass through TeX.

LaTeX uses a file with the same name as your document and the extension .toc to hold the table of contents entries. You can control the level of detail in your table of contents by setting the \secnumdepth counter. A value of zero includes only chapters; one includes chapters and sections; two includes chapters, sections, and subsections, and so on.

The LaTeX commands \listoftables and \listoffigures perform the same functions as \tableofcontents for lists of tables and figures. They use external files with the extensions .lot and .lof, respectively. As with the table of contents, your document can be correctly formatted in one pass if the \listoftables and \listoffigures commands are placed at the end of the document.

Figure references are a special case of LaTeX's cross referencing mechanism. The LaTeX command \label{string} creates a referent. You refer to the label with the command \ref{string}. In normal body text, the label refers to the current section or subsection. In a figure or table environment, the label refers to that figure or table.

If your document contains no forward references (if all \label commands occur before the \ref's that refer to them) then it can be formatted in one pass. Otherwise, TeX will have to be run two or three times to make all of the references correct.

Indexes and glossaries differ from the preceding forms of reference in that they must be processed by a separate program. In general, this is true regardless of the macro package or format you use. An external program is required because indexes and glossaries must be alphabetized, and in indexes, consecutive page numbers have to be converted into ranges, and so on.

LaTeX works in conjunction with another program, called BibTeX, to provide a flexible, convenient way to construct bibliographies. The \cite commands allows you to refer to other documents in much the same way that the \ref command allows you to refer to other portions of the same document.

You make a citation by placing the command \cite{string} where you wish the citation to occur. The string is a key that refers to the document in your bibliography database that you wish to cite. Example  Example 1.2 is a typical entry in a bibliography database. It describes Knuth's classic book The TeXbook [kn:texbook]. The key for this entry is “kn:texbook.”

@Book{kn:texbook,
  author    = "Donald E. Knuth",
  title     = "The {TeX}book",
  publisher = "Addison-Wesley",
  year      = 1989,
  edition   = "Fifteenth",
  isbn      = "0-201-13447-0"
  note      = "Paperback ISBN: 0-201-13448-9"
}

Each entry in the database consists of a type (book, article, magazine, etc.), a key, and a number of fields. The number and names of the fields depend on the type of entry. The database is simply a plain ASCII file containing any number of entries. You can have multiple databases.

These are the commands you use, in addition to \cite, to include a bibliography in your document:

\bibliographystyle{plain}
\bibliography{textools,refbooks}

The \bibliographystyle command tells BibTeX how to format the bibliography, and the \bibliography command identifies which bibliographic databases contain the citations that you have made. The “plain” style of bibliography is selected, and the textools and refbooks files contain the bibliographic information for the documents cited. Document styles can be used to alter the format of citations in your text. The default extension for bibliographic styles is .bst. The default extension for database files is .bib.

LaTeX places citations and bibliography information into the .aux file. BibTeX reads the .aux file and constructs a bibliography, which it places into a file with the extension .bbl, using the entries you cited and the bibliography style you selected.

Sometimes, producing a complex document requires the ability to interface with objects outside of TeX (pictures or figures created by high-end graphics packages, special features of a particular printer, etc.). To support this kind of communication, TeX provides a control sequence called \special. The arguments passed to the \special command are written directly to the DVI file for the DVI driver. It is the responsibility of the DVI driver to handle them. DVI drivers typically ignore \special commands that they do not recognize.

You will find \special commands of various kinds described throughout this book, particularly when discussing color typesetting in Chapter Chapter 4, Chapter 4, and graphics in Chapter Chapter 6, Chapter 6.