Most of the 9V NiCd or NiMH batteries I've taken apart use flat (oval) coin cells stacked on one another (and usually separable.) Frequently 7 of them (8.4V) rather than 6. (The AAAA configuration only works with 6 cells.) There's actually a surprising amount of variation in the internals of 9V batteries... (9V "ultra" lithium batteries seem to have a single molded plastic module with three internal cells. A shame that they're not separable into 3V cells; those might have filled a nice niche in the battery space. (bigger than coin cells, smaller than camera batteries...)