Historians often encounter striking correlations between events in different places at the same historical period: a new disease breaks out in several towns in the same month, for example, or a new phrase comes suddenly into widespread use. Sometimes these patterns are merely coincidental, but in general -in history as in ordinary life -we look for an explanation of the correlation in terms of a single common event, with which each of the original events may be seen to be associated. We look for the original source of infection, or the original use of the phrase. In any particular case, it may not be obvious where we should look for the linking event, and historians need a nose for these things. But one piece of advice seems infallible: Look in the past, not in the future. In other words, look for the linking event before, not after, the events it is supposed to explain.