The online description said is was an ‘elements of style’ for cobol. I suppose that applies to cards too.

Jerry was asked about this yesterday (12/4) in the Q&A after his re:Clojure keynote. Specifically, he was asked about the hat he wore. He said it was because when he and Hal taught 6.001 at MIT, the eval/apply lecture was a special one, with magic acts, small explosions, etc., and he and Hal wore different magicians’ hats.