If you have even a hint of curiosity in your soul, you cannot help but wonder why there are AA, C and D batteries, but no "B". Well, it turns out that there were B Batteries, which were used for things like radios. Vintage radio enthusiasts can find instructions for making your own B Batteries here. An antique radio site explains what they were used for:
It will help to understand a little about the difference between the A and B power supplies.
The A supply provides low-voltage DC to heat the filaments inside the radio tubes. It can be as low as 1.5 volts.
The B supply provides higher-voltage DC for the "plate" circuits of the radio. The B supply can be 22.5, 45, 67.5, or 90 volts.
Why the difference in voltage between A and B? The answer has to do with the way that tubes work.
When you connect the A battery, the filament of the tube is heated to release negatively charged electrons. When the B battery is connected, it puts a positive charge on the plate of the tube. Electrons travel through the partial vacuum inside the tube, flowing from the filament to the positively charged plate. Many tubes also have small structures, known as grids, between the filament and the plate. The grid regulates the number of electrons that strike the plate.
Thus, every radio tube must be supplied with two different voltages (A and B), and most will need three (A, B, and C). The A voltage heats the filament to release electrons. The B voltage gives the plate a positive charge to attract electrons from the filament.
All early radios apparently used batteries, often getting recharged by the car. Then technology changed:
Early battery sets had several drawbacks. A dead battery could leave you radio-less in the middle of a crucial broadcast. Lead acid cells could leak acid, which might drip out of the radio cabinet onto your lovely Persian rug. Worst of all, if you accidentally reversed the A and B battery connectors, you could fry your radio's precious tubes.
Recognizing these problems, radio makers, many of which also made and sold tubes, sought to develop battery-less radio sets. Perhaps more radio tubes could do part of the job of expensive disposable batteries.
Radio tubes offer two important features. A tube can act as an amplifier, taking a tiny voltage, such as an incoming radio wave, and increasing it sufficiently to be heard through a headphone or speaker. A tube also can act as a diode, which changes alternating current (AC) into a series of half-cycle pulses that approximate the direct current (DC) that flows from a battery.
Alternating current became increasingly available in homes during the 1920s, and radio engineers soon developed new radio tubes, called rectifiers, which could convert AC to DC. Soon, the stores were filled with battery-less radio sets using rectifier tubes. Everyone loved these new radios, except the battery manufacturers.
Eventually, of course, the transistors entirely eliminated the need for the heavy voltage B batteries provided. The B battery seems to have fallen out of the ANSI standards, which started being formulated in the late 1920s, as did the A battery. So now you know why there's a letter gap on the drugstore shelf.
The act of creating standards is always going to create these sort of little artifacts; once people start using them widely, you can't revise away the gap when it turns out that no one is using that battery size any more. It's only been a century since we started seriously standardizing, and already our naming regimes have a lot of these "missing" items. Presumably, as the centuries march on, they will be littered with these hole-shaped relics of earlier technologies.