What is the best way to customize the headings at the top of each page in a memoir document? For example, if I want just the page number at the top of every left page, and just the chapter title (say, in italics) at the top of every right page? What if I don't want any header on left pages, but just the page number as a footer in the corner?

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The best way would be to use memoir's header typesetting styles. :) This is described in detail in the memoir documentation (section 7.2 Page styles, starting on p 118). –  Werner Dec 16 '11 at 0:38
There is also my PracTeX Journal article about the memoir page styles: tug.org/pracjourn/2008-2/madsen –  daleif Dec 16 '11 at 9:51

As Werner comments, the best way to customize the headers/footers when using memoir is to use the features provided by the class itself.

In the following code I created a customized page style to produce your first layout: page number at the top (I chose at the left) of even-numbered pages and chapter title in italics at the top (I chose at the right) of odd-numbered pages; since I didn't make any special provision for the plain style, the first page of every chapter will retain the default plain style:

\documentclass{memoir}
\usepackage{lipsum}

\makepagestyle{mystyle}
\makeevenfoot{mystyle}{}{}{}
\makeoddfoot{mystyle}{}{}{}
\makepsmarks{mystyle}{%
\createmark{chapter}{left}{nonumber}{}{}}

\pagestyle{mystyle}

\begin{document}
\chapter{Test Chapter}
\lipsum[1]
\section{Test Section}
\lipsum[1-20]
\end{document}


Here are the modifications required in the comments; the headers/footers will now extend to the space reserved for marginal notes; the plain style has also been redefined to match the layout for other pages:

\documentclass{memoir}
\usepackage{lipsum}

\makepagestyle{mystyle}
\makeevenfoot{mystyle}{\thepage}{}{}
\makeoddfoot{mystyle}{}{}{\thepage}
\makeatletter
\makepsmarks{mystyle}{%
\createmark{chapter}{left}{shownumber}{\@chapapp\ }{.\ }}
\makeatother

\makepagestyle{plain}
\makeevenfoot{plain}{\thepage}{}{}
\makeoddfoot{plain}{}{}{\thepage}

\pagestyle{mystyle}

\begin{document}
\chapter{Test Chapter}
\lipsum[1]
\section{Test Section}
\lipsum[1-20]
\end{document}

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After reading and experimenting around, I came up with the following (closest version to what I want): \nouppercaseheads \makepagestyle{mystyle} \makeevenhead{mystyle}{\itshape\leftmark}{}{} \makeoddhead{mystyle}{}{}{\itshape\leftmark} \makeevenfoot{mystyle}{\thepage}{}{} \makeoddfoot{mystyle}{}{}{\thepage} \makepsmarks{mystyle}{\createmark{chapter}{left}{}{}{}} But here the headers on each page say 1Test Chapter. What is the best way to get them to say Chapter 1: Test Chapter (still in italics)? Thanks! –  jamaicanworm Dec 16 '11 at 16:31
@jamaicanworm: see my updated answer. –  Gonzalo Medina Dec 16 '11 at 17:20
@jamaicanworm: do you want only the footer on the first page of chapters or also the header? –  Gonzalo Medina Dec 16 '11 at 20:24
@jamaicanworm: see again my updated answer. –  Gonzalo Medina Dec 16 '11 at 21:07
@jamaicanworm: You're welcome! –  Gonzalo Medina Dec 17 '11 at 2:42