Russian is ambivalent in this way. For example, they say "on the plain" but "in the field." It's just generally accepted usage, not a rule. Sometimes the choice

Russian is ambivalent in this way. For example, they say "on the plain" but "in the field." It's just generally accepted usage, not a rule. Sometimes the choice depends on a local or family's tradition, even: some say "in the hallway" but "on the kitchen" while others would say it the other way around (either way they would never use the same preposition, strangely enough).