Apple LaserWriter 12/640 PS - Manual (Page 206)

Printing on both sides of the paper
With the optional duplex printing unit, you can print on both sides of the
paper (often called duplex printing or two-sided printing.) You can only duplex
print from the paper cassettes.
If you want to You need this much printer memory
Turn on PhotoGrade or use the 12 MB
duplex printing option
Turn on PhotoGrade and duplex 20 MB
print on all size paper (except legal)
Turn on PhotoGrade and duplex 24 MB
print on all size paper (including legal)
Note: You cannot duplex print from the multipurpose tray. You cannot print
to the face-up output tray when duplex printing.
1 Choose Print from the File menu.
The Print dialog box appears.
2 Choose Layout from the Settings pop-up menu.
The Layout dialog box appears.
181 Mac OS Users