What was the Leitner box system, and why does it matter here?

A low-tech paper ancestor of spaced repetition: flashcards moved between numbered boxes on success or failure.

Sebastian Leitner popularised it in the 1970s. Cards sit in boxes reviewed at different frequencies. Get a card right → promote it to a box reviewed less often (a longer interval). Get it wrong → demote it back to box 1 (reviewed most often). Hard cards naturally get seen more; easy cards drift toward the back.

It's the same core idea every modern algorithm automates: success stretches the interval, failure collapses it. The software just replaces cardboard boxes with a per-card number.