HP 4100TN - Manual (Page 53)

EN Printing both sides of paper (optional duplexer) 51
Placement for paper requiring a specific
orientation
Several kinds of paper require a specific orientation when you
duplex—for example, letterhead, preprinted paper, and paper with
watermarks and prepunched holes. The duplexer prints the second
side of paper first. Orient paper in the trays as shown below.
For tray 1, load the front side facing down and the top toward you. For
all other trays, load the front side facing up and the top toward the
printer.
Tray 1 Trays 2, 3, 4