The 2-sided printing feature is not available for the rear paper feed slot.
Copy paper, Plain paper (except for A3, B4)*1
Letter head (except for A3, B4)*2
*1:For automatic 2-sided printing, you can use A4, B5, 16K, Letter, User Defined (101.6×152.4 to 215.9×297mm: 4×6 to 8.5×11.7 in.) paper sizes.
*2:Manual 2-sided printing only.