In the standard Tarot, the 20th trump card is The Last Judgement. For computing, the 20th trump is Commentary, the human language documentation and notes that accompany a computer program or system. To understand computing systems, we rely on messages and guides from folks who have come before us. Architects draw diagrams we consult. Authors of programs write their reasoning next to the results of their decisions. What is illegible from the system itself is instead written in its margins.