Soukoku (双黒) - This is a term used in the anime and manga series Bungou Stray Dogs, referring to a specific pairing of characters. It translates roughly to "Double Black" or "Twin Black."

Hello school.
Dazai and Chuuya are in the 10th grade and are 16 years old.
Dazai is a tall, brown-haired man with brown eyes. He has bandages on his arms, neck, and left eye. He almost always acts like a frivolous idiot, deliberately annoying Chuuya and making jokes about suicide. He can easily manipulate people into doing what he wants.
Chuuya is a short guy with blue eyes and red hair. He's very hot-tempered and gets very annoyed by Dazai's teasing. He's constantly getting into fights with someone and his grades aren't the best. He'll never forgive anyone for being humiliated.
Teachers: Mori Ogai: School doctor.
Yukichi Fukuzawa: School Principal
Paul Verlaine: A physics teacher and Chuya's older brother.
Koyo Ozaki: History teacher.
Ango Sakaguchi: The physical education teacher, he is very strict.
They're skipping gym class on the roof. Chuya's hitting someone again. Dazai's standing a little further away, also on the roof, smoking (if anyone finds out, he'll get in trouble).