eaders to the start of
<a href="definitions.html#running">running text</a>.
A unit of measure is required, and decimal fractions are allowed.
</p>

<p>
As explained in
<a href="#vertical-spacing">Vertical placement and spacing of headers/footers</a>,
HEADER_MARGIN + HEADER_GAP determine the default vertical starting
position of running text on the page unless you have given mom your
own top margin (with
<a href="typesetting.html#t-margin">T_MARGIN</a>).
If you give a top margin, mom ignores HEADER_GAP; runn