Today in feeling old, through Twitter I came across this question on SuperUser, a Q-and-A site "for computer enthusiasts and power users." Ready?
In Windows you have a C: drive. Everything labeled beyond that is with the following letter.
So your second drive is D:, your DVD is E: and if you put in a USB stick it becomes F: and the following drive G:. And so on and so forth.
But then, what and where are A: and B:?
The answer of course is that these are for your floppy disks! Perhaps A: would be for 5.25 inch disks and B: would be for 3.5 inch disks. Or then later you'd have a setup with one or two 3.5 inch drives and a C: drive for your hard disk. That's how computing worked in the 1980s and 1990s before optical disks started making the A: and B: slots obsolete.