When the Republican nominee launched his presidential campaign 17 months ago it seemed more like a publicity stunt by a reality-TV star than the first step to the White House. Yet on election day opinion polls, betting markets and forecasters suggest that Donald Trump has a 20-30% chance of becoming the 45th president. Another way to put this is that a Trump presidency is more likely than rolling a six on a dice, and about as likely as the Cleveland Cavaliers (the second-best team, according to bookmakers) retaining their title in the National Basketball Association final, or rain on any given day in London. A grimmer analogy, perhaps, is with Russian roulette: a Trump win is rather more likely than a bullet in the head from a six-shooter. It probably won’t happen. But history is made by low-probability, high-impact events, and a Trump win would certainly be that.