Left margin wider than right one

Question

Why is the left margin wider than the right one? Each of them should be 1 inch wide:

If you run TEX without modifying the plain TEX format, you get pages that are numbered at the bottom; and each page will be approximately 8.5 inches wide and 11 inches tall, including 1-inch margins at all four sides. [The TeXbook, p.251]

\moveleft2cm\vbox{\hrule width2cm}
\moveright\hsize\vbox{\hrule width2cm}

UPDATE

How can the document know the size of paper it is being typeset on?

Answer 1

This should explain the apparent contradiction; compile with pdftex

\pdfpageheight=11in
\pdfpagewidth=8.5in
\moveleft2cm\vbox{\hrule width2cm}
\moveright\hsize\vbox{\hrule width2cm}
\bye

If you use tex+dvips+ps2pdf for producing a PDF file, change the first two lines into

\special{papersize=8.5in,11in}

You'll get

The fact is that the standard setup of TeX distributions, regarding PDF output, is ISO A4 paper and not Letter paper.

A rudimentary test for the various situations might be as follows:

\def\dvipsletterpaper{\special{papersize=8.5in,11in}}
\def\pdfletterpaper{\pdfpagewidth=8.5in \pdfpageheight=11in }
\ifx\pdfoutput\undefined
  % we're either with Knuth TeX or XeTeX
  \ifx\XeTeXversion\undefined
    % we're with Knuth TeX
    \dvipsletterpaper
  \else
    % we're with XeTeX
    \pdfletterpaper
  \fi
\else
  % we're either in pdfTeX or LuaTeX, we assume \pdfoutput is an integer
  \ifnum\pdfoutput>0
    % PDF output
    \pdfletterpaper
  \else
    % DVI output
    \dvipsletterpaper
  \fi
\fi

Answer 2

In traditional typography for twosided printing, the inside margin has to be half the outside margin, simply because when you open a book, you see two inside margins next to each other, so that you see 3 horizontal white spaces of equal width.

Of course one also has to take into account a binding correction, so that the most common formula, known in France as the "Canon des Ateliers" — canon here meaning model, is the following: a fraction of paper width is reserved for the printing zone (most of the time 3/4 for ordinary printing, 2/3 or even 5/8 for high quality). The rest is white space, shared according to the proportion: 4:5:6:7.

This means that inside margin gets 4/10 of the white horizontal space, outside margin gets 6/10. 5/10 is for top margin and 7/10 for bottom margin (which makes a total of 12/10 for vertical white space).

Answer 3

Q: How can the document know the size of paper it is being typeset on?

Traditional TeX doesn't know the papersize. The pagebox is positioned to the point 1in from left, 1in from top of hypothetical paper: left/upper corner of the pagebox is here shifted by \hoffset right (or left, if negative) and \voffset down (or up if negative). This point (1in,1in left,top) is an origin of the typesetting of traditional TeX regardless of the actual papersize. May be the actual papersize can be smaller than pagebox and you'll not see all typesetting without any overfull box warnings.

pdfTeX is able to set the media size (i.e. papersize) by \pdfpageheight and \pdfpagewidth. The origin of the typesetting mentioned above as 1in,1in is possible to change in pdfTeX by \pdfhorigin and \pdfvorigin (they are set to 1in,1in by default).

If you set \pdfpagewidth then you can calculate \hsize as \pdfpagewidth minus left margin minus right margin. Moreover you have to set the sum of \pdfhorigin plus \hoffset as the amount of the left margin. The analogical princile is applicable for vertical margins.

The registers mentioned above have to be set before \shipout (shipping out the page). They can be changed to another values for another pages.

Edit: PostScript language uses different origin of its default coordinate system than TeX: 0,0 left,bottom of the paper. This implies that dvitops convertor needs to set the new origin and it needs to know the actual paper height. The paper width is irrelevant for this need. Moreover PostScript can set the media size by setpagedevice operator (4.11 in Redbook). The dvitops convertor can get the data about media size from \special or from command line or from configuration file. The data are used by setpagedevice operator in dvitops output.

The PostScript or PDF documents have the media size set. When this document is printed on the printer with different media size of paper sheets loaded in the printer then printer warns or makes trouble to the user.