Blue wall of silence
（重定向自Blue Code）
The blue wall of silence, also blue code and blue shield, are terms used in the United States to denote the unwritten rule that exists among police officers not to report on a colleague's errors, misconducts, or crimes. If questioned about an incident of misconduct involving another officer (e.g. during the course of an official inquiry), while following the code, the officer being questioned would claim ignorance of another officer's wrongdoing.