 The ubiquitous AA and AAA batteries each provide about 1.5 volts. When you need more voltage, you connect them end-to-end in series. So a 4-cell case like this will supply 6 volts. Even larger alkaline cells like these D batteries still supply about 1.5 volts. So what's up with 9-volt batteries? What makes them able to supply 6 times as much as a regular alkaline battery? Well, one way to find out is by opening it very carefully. And that's about the simplest answer. A 9-volt is really 6 very small 1.5-volt cells wired in series. Makes sense? It's like they're standing on each other's shoulders with a big trench coat on.