system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do batteries always go the opposite way round to each other?
gpt
Generally batteries are hooked up in series — that is, power flows from the positive end of one battery to the negative end of the other battery, then out to through the device and back into the negative end of the first battery. (Actually, it's the reverse of that, but that's irrelevant to our discussion).

Given the batteries being hooked together like this, putting one in “backwards”, so to speak, lets you connect the two batteries on one end of the case with a short bit of wire, and run from the other ends of the batteries to the device in a more efficient layout.