The McDonald's McRib is back, hitting restaurants nationwide today.

The legendary boneless pork sandwich was supposed to return at the end of October, but was pushed back to help boost end-of-the-year sales.

Better late than never.

The limited-time product, famously molded to resemble a rack of ribs, is both a feat of modern engineering and shrewd marketing.

It garners almost as much attention for its pseudo-meat shape as its impermanence on restaurant menus.

How did the pork patty, barbeque-sauce-slathered concoction originate? And why is it always disappearing from menus?

UPDATE: McDonald's representative Becca Hary has emailed us with some corrections reflected in a few of the following slides.