I’m sure you’re referring to these Opalithplättchen, aren’t you?

Impressive that you could remember the name. I’m German and had to look it up since I never heard about Opalith before.

Exactly! Bicycle pedals have a left-hand thread on the left-hand side and “normal” threads on the right-hand side.

In the plumbing sector, left-hand threads are used whenever two pipe ends need to be connected that cannot be rotated. The connector is then equipped with a left-hand and right-hand thread and can therefore easily be screwed between them.

So it’s not just typical for Nordic countries, but depends on the application.