[alecmg]

In several cars the air vents are marked with filled circle and empty circle and I always struggle to remember which means which.

[perl4ever]

I was looking at the dials on a refrigerator, and realized I never really knew which way is which - does turning the dial towards "cold" make it colder, or does the opposite, revealing more of the indicator from the "cold" end make it colder?

There is a tapered blue line with numbers, so is the wider end colder, because blue is cold and more blue is colder...or is the other end colder, because it has smaller numbers, and lower temperatures are smaller numbers?

[Slartie]

Filled circle means "vent is blocked, so no air can pass". Empty circle is an unblocked vent through which air can pass.

[gmac]

I think the problem is those meanings can plausibly be reversed, so it's hard to remember which way round it goes.

A better way to do it that I've seen is to represent air flow with wavy lines or similar on the 'open' symbol.

[alecmg]

Pretty sure in the last car it was the opposite. Filled circle meant air was coming.

So you can see why this is confusing