While Americans across 50 states and Washington, D.C. are voting on Election Day—or already voted early—some states’ votes will be more closely watched than others. Over the last half-century, the share of Americans who vote for the same party in every election has increased; unsurprisingly, some states are consistent in just the same way. And so all eyes will be on the handful states that do vary their votes: the swing states.

So how did those states get to be so influential?

Before the 1970s, one could find rare, sporadic references to the term swing state, but back then it meant something more like a bellwether state—a state that political journalists looked to because they believed its residents would vote for the eventual winner of the Presidential election. “The phenomenon has been around for along time but had a different label,” says Alex Keyssar, author of Why Do We Still Have the Electoral College? and professor of History and Social Policy at Harvard University. Political pundits used other terms to describe swing states, such as “doubtful” states.

The 1976 election, when outsider Georgia Governor Jimmy Carter defeated incumbent Gerald Ford, was rife with swing states, Keyssar says, in part because it was still in the period when many Southern Democrats were shifting to the Republican Party. That situation contributed to a situation in which a high number of states could have gone either way.

But the difference between now and then was about more than just vocabulary and timing.

“The two political parties had been more ideologically diverse, so you had less polarization,” says David Schultz, an editor of the anthology Presidential Swing States: Why Only Ten Matter. “As polarization becomes more rigidified since the 1970s—with people less likely to split-ticket vote; people less likely to vote Republican in one election, Democrat the next—that’s kicked in the notion that states have also more firmly become partisan one way or another.”

Schultz and his co-editor Stacey Hunter Hecht define a swing state as one that has frequently boasted a 5% or smaller difference in votes for the two major candidates, a vote result that often matches the national popular vote, and a history of flipping between parties. With the election of George H.W. Bush in 1988, the swing-state situation had started to evolve to the way things stand today, with just a handful of states as perennial battlegrounds.