What is the difference between "low profile" and "low profile ready" in expansion cards?

"Low Profile cards are usually half the height of regular cards and come with a small bracket that doesn't fit into a standard slot, you need a specific slot.

Low Profile ready cards come with a smaller bracket as well, so you can choose to install it on a regular or low-profile system.

The latter is best if you are unsure and want flexibility in the future."

In my experience, "low profile" means the card itself and the metal slot cover on the edge of the card both come in the "low profile" form factor out of the box. "low profile ready" usually means the card is low profile...but the slot cover might not be. If the vendor is nice, they'll include a low profile slot cover that you can switch to. But you might also have to get one separately.

"Why did the industry make the two brackets with notched ends on opposite sides?
It pretty much eliminates the use of slot adapters which do exist (Startech makes both PCI slot to PCIe card and also PCIe slot to PCI card). But they are uttlerly useless if you cannot use the low profile bracket that comes with the card you want to plug into the converter."