Former vice president and presumptive Democratic presidential nominee Joseph Biden has announced U.S. Senator Kamala Harris as his running mate in the 2020 presidential election. Though women have been nominated for the vice presidency, Harris is the first Black woman and first person of Indian descent to be nominated by a major political party. No woman has ever been elected vice president.

At any moment, the vice president of the United States could become the world’s most influential leader. But the role of vice presidents has evolved dramatically through the years, from irrelevant throughout much of history to a potential instrument of power today. Here’s what you need to know about the position.

The role of vice president was a governmental