When you flip the floppy over the media rotates the other way. Supposedly the inside of the jacket had a fabric coating who's purpose was to catch fragments of media falling off during use. If you spin the media the other way those fragments come back off and gunge up the head. Or so the story goes.

Also of course there were real double-sided drives that used both sides of the media (with two heads). Vendors expected users to use those and pay extra.

Source: punched many a hole in Pet floppies back in the day.

There was indeed a fabric coating in most floppies, but in my experience taking them apart I never found any material that looked directional.

https://upload.wikimedia.org/wikipedia/commons/thumb/2/21/8-...

I have a fabric brush that works on that principle, such that I have to take care when using it left-handed instead of right-handed as the direction changes and instead of removing lint from my garment it puts all the accumulated lint back on if I run it the wrong way.

But this is almost certainly not the case with the floppy disk fabric. Virtually every piece of Apple software sold on floppy that needed more than 1 disk side used the flip-side of the disk, making disk-flipping and running the disk "backward" not only common but necessary almost every time you used the computer.

There were floppies sold as double-sided and they came at a premium price: Before the floppy would be sold, it had to be verified at the factory and having both sides of the media functional was harder than just one side.

To expand: Early on there might've been some differences in the media rated for double sided disks, but by the time the floppy disk was a consumer item they were all using "good enough" media, and you could hole punch any SS disc to use the other side in an SS drive.

Few cared much really because the double sided drives were clearly the next one you were going to buy even if you hadn't got one yet. And then it wouldn't matter.

A fun trick with some combinations of drive and system involved having a disk that could be read differently when flipped, in a double sided drive. It wasn't viable large scale but crazy people having fun with stupid floppy tricks might produce a few occasionally and hand them around.

(Involved halving or less the sector counts and abusing alignment to interleave one side's sectors beside the others. If you wanted to be evil there were further refinements in what you could make the system report was there that wasn't)

I remember being told this was actually a myth. The manufacturers claimed this to be the case, but the problem was that different systems' drives actually used different sides of the disk, so the disk manufacturers had to verify both sides anyway.

Newer floppy drives had heads on both sides, you didn't need to flip the floppy and single write protection hole were used for both sides.