Even further back into time, before 3.5" disks, both A: and B: were 5.25" disks. And, although my memory is hazy, 3.5's were commonly slotted into B: at first, because no one had 3.5" boot disks until the drives became somewhat common.

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gumby 2 hours ago | parent | next [-]

You are forgetting 8” floppies. IBM used only the soft sector ones (one hole punched close to the spindle to mark sector 0) but there were also hard sector ones (a ring of 32 holes close to the spindle hole).

There was a lot of experimentation back in those days. The Wikipedia’s page on floppy disks is surprisingly long!

	▲	LukeShu 23 minutes ago \| parent \| next [-]
		But those weren't on "IBM PC"-compatibles, just older micros? The 5150 had 5¼" drives. That is: (Q\|MS\|PC)-DOS never supported 8" floppies, right?
	▲	jasomill an hour ago \| parent \| prev \| next [-]
		Floppies, superfloppies, removable hard disks (platter-only, as opposed to modern external hard drives that incorporate the entire drive mechanism), magneto-optical disks, rewritable CD/DVD, even today magnetic tape is still used as removable storage in applications where large capacity is required and shelf life is more important than fast random access.
	▲	compiler-guy an hour ago \| parent \| prev [-]
		Trust me, I will never forget 8" floppies. But in the context of drive lettering, although CP/M supported 8" floppies, MS-DOS never did. MS-DOS adopted the naming scheme, but not the boot style.

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raldi an hour ago | parent | prev [-]

The floppies got A and B because hard drives were expensive and for a time a lot of people got by without them, though everybody had at least one floppy.

Even if you only had one floppy, it was both A and B, so you could say "copy a:DOC.TXT b:" and it would read the doc and then ask you to insert the floppy you're considering B (and depending on file size and available memory, sometimes switch back and forth a few more times)