\headheight
book
class, where it varies with the
type size.
\headsep
book
class, where it varies with the type size.
\footskip
book
class, where it varies with the type size.
\linewidth
article
document in
‘10pt’, it's set to ‘345pt’; in two-column mode, that
becomes ‘229.5pt’.
\textheight
article
or
report
document in ‘10pt’, it's set to
‘43\baselineskip’; for book
, it's ‘41\baselineskip’.
For ‘11pt’, it's ‘38\baselineskip’ and for ‘12pt’,
‘36\baselineskip’.
\textwidth
article
or report
document, it's
‘345pt’ at ‘10pt’, ‘360pt’ at ‘11pt’, and
‘390pt’ at ‘12pt’. For a book
document, it's
‘4.5in’ at ‘10pt’, and ‘5in’ at ‘11pt’ or ‘12pt’.
\topmargin
\paperheight − 2in −
\headheight − \headsep − \textheight − \footskip
, and
then divided by two.
\topskip