Toilet Door: What do Triangle and Circle mean?

I recently saw a toilet that had one door labeled with a triangle and the other door labeled with a circle instead of the typical male and female pictograms. I was completely unsure which toilet could be behind which of these doors and decided not to go to the toilet at all because I didn't want to blunder into an unpleasant situation.

So that this will not happen to me again: Can someone explain to me which symbol represents which gender?