Running heads and feet

A running head is the strip of text at the top of each page, and the running foot is its counterpart at the bottom. In a book or thesis they usually carry the chapter title and the page number; in a submitted paper they might carry a short version of the title, the authors, or nothing at all. LaTeX’s built-in page styles (plain, empty, headings) cover the basics, but the moment you want a specific combination, such as your title on the left and the page number on the right, you reach for the fancyhdr package.

The standard recipe

\usepackage{fancyhdr}
\pagestyle{fancy}
\fancyhf{}
\fancyhead[L]{Short title}
\fancyhead[R]{\thepage}
\renewcommand{\headrulewidth}{0.4pt}

Each line here does one job. \usepackage{fancyhdr} loads the package in your preamble. \pagestyle{fancy} switches the whole document over to fancyhdr’s configurable style. \fancyhf{} clears every header and footer field, which matters because fancyhdr ships with defaults (page number and section marks) that you almost never want mixed with your own; starting from a clean slate keeps surprises out.

The next two lines fill fields back in. \fancyhead[L]{Short title} puts literal text in the left slot of the header, and \fancyhead[R]{\thepage} puts the current page number in the right slot. \thepage is a command that expands to the page number, so it updates on every page automatically. There is a matching \fancyfoot[L/C/R]{...} if you would rather put things at the bottom.

Finally, \headrulewidth controls the thin horizontal line under the header. Setting it to 0.4pt draws a hairline; set it to 0pt to remove the rule entirely. A \footrulewidth exists too and defaults to zero.

Two-sided documents

If your document class uses twoside (the default for book), pages alternate between even and odd, and headers usually mirror each other. fancyhdr handles this with even/odd selectors: \fancyhead[LE,RO]{\thepage} puts the page number on the left of even pages and the right of odd pages, which is where a reader’s thumb finds it. You can combine this with \leftmark and \rightmark, which hold the current chapter and section titles, to reproduce the classic book layout.

Chapter pages and the empty look

Chapter opening pages ignore your fancy style because \chapter calls \thispagestyle{plain} behind the scenes. That is deliberate, and most designs keep it. If you want those pages bare, add \fancypagestyle{plain}{\fancyhf{}} to redefine what plain means.

One common warning to expect: fancyhdr may complain that \headheight is too small. The fix is exactly what the message suggests, for example \setlength{\headheight}{14pt} in the preamble, or passing headheight=14pt to the geometry package if you already use it. It is a layout reservation, not an error, but silencing it keeps your log clean and your baseline grid honest.