From The Social Order of the Underworld: How Prison Gangs Govern America’s Penal System by David Skarbek:
The difficulty of corresponding between prisons makes reaching agreement especially costly. Officials forbid inmates from sending mail to or calling each other. They can use pay phones, but officials often monitor and record these conversation. Many gang members learn obscure languages to obfuscate their discussions, such as the ancient Aztec language Nahautl. Inmates use “micro writing,” letters written smaller than a quarter of inch. [...] Officials are not allowed to read legal paperwork, so inmates often write letters that appear to be legal work and slip the messages in with legitimate documents. [...] A creative inmate might also write a message in his own urine. After it dries, it becomes invisible, but once the recipient heats the paper, the hidden message returns. Though effective, these forms of communication are costly.
Because of the high cost of communication, even a supreme gang leader who wants to direct all activity at all prisons would find it useful to decentralize command to some degree.