black step
A heavy line, 6 points thick and about 24 points
long, which appears on the back of a folded
section of a book after the printed sheet is
folded. The collective black steps run from the
head to the tail of the gathered sections,
generally about 24 points below each other. When
the book is gathered the black steps appear as a
diagonal line running from head to tail across the
back of the text block, and any missing or
misplaced section immediately becomes obvious
because the black step is broken. Also called
"back mark," "collating mark," and "quad mark."
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