The Tristero is a fictional postal service featured in the novel "The Crying of Lot 49" by Thomas Pynchon. It operates as an underground network, symbolizing alternative communication methods outside of mainstream society. The service is shrouded in mystery and is associated with a conspiracy that challenges the reliability of established systems. In the story, the protagonist, Oedipa Maas, discovers the Tristero while investigating the estate of her deceased ex-lover. The service's emblem, a muted post horn, represents a hidden world of messages and connections, reflecting themes of paranoia and the search for meaning in a fragmented reality.