» The reason you should determine the text width first is that the width of a
» line is very important for how easy a text is to read: if the line is too
» long then it is hard to find the beginning of the next line.

but designers don't (necessarily) work that way: you usually have the
constraint of the paper format (or choose one in the first place),
then choose the text width/height and adjust type size/leading to
achieve nice text blocks. You can't say like in current standard
classes `i want that font size/leading on that paper' and get a line
length/textheight computed, because you'd adjust the leading depending
on the line length, and the line length (together with type size)
depending on the margins...

» For many of
» the common paper formats (e.g. A4) it is more often this than the
» \paperwidth that is the bound you need to consider.

it is more or less impossible to achieve a nice layout on iso paper
anyway...

» The text height and width are the side lengths of the text rectangle, in
» which all the non-marginal text should appear. In traditional book design,
» the size and position of this rectangle is one of the very first things you
» determine.

yes, and find a font/leading that fits afterwards...

» That's the way the current output routine does it, yes, but it is not the
» way it should be done. E.g. a headings pagestyle page head is visually part
» of the text rectangle

they're not! in fact it depends: when you have a rule under the
heading, very close to the body of the text, you consider the heading
as part of the text block, but usually, you don't. If you have folios
at the bottom of the page, they're definitely out of the text block. I
think Tschishold says something about that, but maybe I remember it
wrong.

» Another thing which should be included in
» \textheight is the (expected) depth of the page box; I doubt anyone would
» want to claim that the descenders on the last line of a page are outside

I would. I mean that what designers choose (often with a cryptic
combination of oblique lines) is the rectangle that will apear grey in
a typical page: its top is a x-height (or cap-height) over the first
base-line, its bottom is the last baseline. In TeX, topskips
have to be quite large when you use accented caps e.g., but the real
visual text blocks starts somewhat lower.

Thierry Bouche
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  « Ils vivent pour vivre, et nous, hélas ! nous vivons pour savoir. »
                                Charles Baudelaire, Paris.