Guide to LaTeX (4th Edition) (Tools and Techniques

16.3. A model letter customization 363
\renewcommand{\@evenhead}{}\renewcommand{\@evenfoot}{}}
This page style must (re)define the commands \@oddhead and \@oddfoot,
which are always inserted at the top and bottom of odd pages, to place
our special \firmhead and \firmfoot. The even pages are unimportant,
since the first page is always odd. It is necessary to enlarge \headheight
and \headsep to allow the big boxes to fit in.
Subsequent pages are set with the headings or plain page styles. We
want to modify the former to include the firm address once more.
\renewcommand{\ps@headings}
{\setlength{\headheight}{41pt}%
\renewcommand{\@oddhead}
{\parbox{\textwidth}{\usebox{\firm}\\[5pt]
\slshape \headtoname{} \toname\hfill\@date\hfill
\pagename{} \thepage\\
\rule[3pt]{\textwidth}{1pt}}}
\renewcommand{\@oddfoot}{}
\renewcommand{\@evenhead}{\@oddhead}
\renewcommand{\@evenfoot}{\@oddfoot}}
One small problem remains: the first time one of these page style
commands is executed, the head and foot commands may not yet exist,
causing \renewcommand to complain. We ensure that they are there at
the start by predefining them with \providecommand (Section 8.3.1).
\providecommand{\@evenhead}{}\providecommand{\@oddhead}{}
\providecommand{\@evenfoot}{}\providecommand{\@oddfoot}{}
Now make headings the active page style.
\pagestyle{headings}
There is only one last thing to do, and that is to redefine the opening
command that prints the recipient’s address and the salutation. We add
a bit more, including the personal data of the sender, as well as the
reference information. The address goes in the left field, the personal
data to the right. The references go in a line below the rule, followed by
the subject line. These entries are tested first, and are only included if
they are not blank. Several stored entry commands used here are part
of the standard letter class, such as \toname and \toaddress. The
\@date entry is either \today or whatever text was stored with \date.
\renewcommand{\opening}[1]{\thispagestyle{firstpage}%
\parbox[t]{\leftfield}
{\usebox{\firmreturn}\\
\parbox[b][3.5cm][c]{\leftfield}{\toname\\\toaddress}}%
\parbox[t]{\rightfield}
{\fromname
\ifthenelse{\equal{\telephonenum}{}}
{}{\\ Tel.: \telcode-\telephonenum}