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This 9v battery contained six cells stacked like a layer cake
(lemmy.sdf.org)
Picture of a disassembled Duracell 9v battery. Below the terminal assembly is a clear plastic case where you can see six sets of stacked rectangular terminals and fillings.
Btw, why do the flat ones (4.5V i think?? use 3 round batteries instead of rectangulars? They could have more capacity due to less space wasted.
It's cheaper, especially for such a relatively uncommon battery type.