The contact cards of London-based full-service sex workers, known colloquially as “tart cards,” provide a glimpse into the design of the underbelly—and the underbelly of design. Most prevalent during the 1980s through the early 2000s, these cards were placed inside the iconic red telephone booths that are ubiquitous throughout London as a means of advertising sex work services. The nature of these cards, including their design, materiality, and use have evolved over time in response to changing legislation, social movements, technology, and design trends.