Political conventions have played a unique role in American political history.

In theory, the convention is a conference where party activists come together to choose a presidential nominee.

But in recent years, the parties have managed to narrow down their choice to just one candidate long before the convention begins.

Consequently, they have become stage-managed events, marking the official launch of the presidential campaign of a candidate already selected in the primary contests earlier in the year.

In the past, contested conventions have changed the course of American history - and with the two Democratic candidates running neck-to-neck, their party's convention in August may do so again.

Conventions can reveal the disarray within political parties - most notably, at the Democratic convention in Chicago in 1968....