20-pins on 24-pin board and vice-versa

1. And I mean a power supply with no 4-pin extender, and a motherboard without extra 12-volt power connector. Just a plain motherboard with a 24-pin socket, and a plain PSU with a 20-pin connector.

2. Can I use a 24-pin connector on a 20pin board?

What will happen if the answer is no to either of these questions? Will the board/PSU be damanged? Will it simply not work?

P.S. Just curious, not gonna actually do it, even though I have the means... Mainly that's why I'm curious, got the idea because of that and am wondering how that would work out.