I'll explain. Suppose there is a furniture-maker somewhere who makes chairs.

Now, due to an equipment malfunction or something, the chair-maker finds it easier to make chairs with lower backs. The rest of the chair -- seat, legs and reinforcing bars -- is fine, but the backs are getting lower.

Things get progressively worse, so each successive chair has a shorter back than the preceding one. Eventually, the chair backs have become non-existent -- i.e., they are stools, not chairs.

If you placed every product in a line in chronological order from the first, high-backed ones to the latest ones with no back at all, where is the transition? Which one do you call the last true chair, and which one is the first true stool?