system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: why is 12 volts very common? Why not 11 or 9 volts?
gpt
The voltages produced by a single cell of a battery is determined by the battery chemistry. For lead-acid, this is ~1.5V. 8 of those cells produce 12V.

In terms of "why not 11 volts?", you can't build an 11V battery from a cardinal number of 1.5V batteries. In terms of "why not 9 volts?", it's simply a matter of the people who standardized battery sizes back in the early 20th Century deciding it that way. In theory, we could just as easily have square batteries that produce 9V instead of rectangular ones that produce 12V if they had decided differently.

Once these standards were established, there's an incentive for manufacturers to simply use what the market offers unless there's a compelling reason to do otherwise.

EDIT: It's actually 8 1.5V cells that produce 12V, not 6.