The Last Word: Shall I, Should I, Will I, Must I?

Late last year, the climate treaty negotiators in Paris were hung up over whether to use shall or should when describing the party nations’ obligations. E.g., Uri Friedman, The Fate of the World Lies in Between Brackets, The Atlantic, Dec. 1, 2015, www.theatlantic.com/international/archive/2015/12/brackets-climate- agreement-paris/418041. The news reports said that shall is mandatory, whereas should is just a guideline (like a changing stoplight in my fair city, I suppose), and many nations would not agree to the treaty if shall were chosen.