What are the black spider memos?

The Guardian have started doing great short explainer videos for some of their long-running coverage. This one on the Prince Charles black spider memos is a great example. Campaigners are increasingly producing short video-explainers, but I think the clarity of the Guardian videos is something to learn from. http://www.theguardian.com/uk-news/2015/mar/20/prince-charles-letters-supreme-court-judgment-due