Brother HL-1800 series - Manual (Page 42)

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For Macintosh user
Change the Pages per sheet setting in Layout.
☛ ☛ ☛ ☛ 1.
Click the 'File' menu, and then select the 'Print' option. You will see a
dialogue similar to the one below:
☛ ☛ ☛ ☛ 2.
Click the "General" drop down box. You will see a drop down list with the
following options:
☛ ☛ ☛ ☛ 3.
Once on the layout page you can enable duplex by selecting the "Print on
Both Sides" checkbox.
From here you have the choice to select either long or shot edge duplexing.
Select the option you require for your print job. See the image below.