I first heard about shūgyō kisoku (official work rules) from a professor at law school: "An employer can change the work rules unilaterally if it is gōriteki," I remember him saying.
I would hear the word "gōriteki," meaning reasonable or logical, thousands more times during my law school years, and later in my career teaching law. It is one of the key concepts underpinning the Japanese legal system. Nearly any gray zone of the law is resolved by the court looking at what is "reasonable." And so it is with changing work rules without the employees' consent.
But it wasn't supposed to be that way. Shūgyō kisoku are the foundational rules of a workplace that decide wages, work hours and much else. That's right: It is work rules that settle these fundamental issues, taking precedence over individual labor contracts. They must be created and promulgated at any workplace with 10 or more employees.