There’s a standard for paper sizes called the ISO 216 international standard, aka A-series paper sizes. You may be familiar with it outside of the American 8.5”x11” letter size. The simplicity, for example, where an A3 sheet is exactly twice the area of A4. One A3 sheet folded in half becomes two A4 sheets. This is intentional: when you fold or divide an A-series sheet exactly in half, the resulting sheets have the same proportions as the prior. The aspect ratio is preserved across all A-series paper sizes.

The iPhone Duo is designed with this in mind. The aspect ratio any way you view it, folded or open, is preserved. Apple design knows a thing or two.