The upcoming primary season in Maine is ushering in yet another innovation in the way the state chooses the party candidates who will appear on the general election ballot. The change now allows voters who aren’t enrolled in a political party to vote in party primaries without having to enlist as a party member.

Until this year, voters have been required to register in the party whose primary they wished to vote and were locked into that party affiliation for the following three months. It’s a change that is one of many that have significantly altered the system since its adoption in 1911 – and an occasion to take a glimpse at how other states engage in the same winnowing out process.

The primary system – that is one in which all voters in a political party get an equal vote in who will run in the general election – has been around so long and is so prevalent in America we take it for granted.

But the United States stands alone in the entire democratic world in allowing direct voter participation in these crucial semi-final contests. Elsewhere, party meetings or caucuses mandate the general election choices. Such was long the case throughout this country.

In Maine, for example, caucuses and conventions of a small percentage of voters deemed delegates designated the nominees. Those participating were perceived to be vulnerable to the influence of party bosses and other hierarchical forces. Such a system prevailed until the early 20th century when Wisconsin, in 1903, adopted the first comprehensive state-wide primary law. More than 20 states had done so by the time Maine voted to go the same way in a citizen-initiative referendum in 1911.

All states and the District of Columbia now embrace a system that affords voters the prerogative of nominating candidates at the state and Congressional level. (Five states, including Iowa and Missouri, retain the caucus system at the presidential level.)

We still have dramatically different systems, however. Indeed, some have virtually abandoned the concept of a partisan – that is, party dominated – method altogether.

Nineteen states – including such ideologically different jurisdictions as Vermont and Texas – have no requirement of party affiliation as a condition for voting in their primaries. Though voters in these states can vote in only one party’s primary at a time, they can choose any party they want on the spur of the moment.

California and Washington state have also dimmed the political parties’ roles. Each of them in recent years has embraced the “top two” primary. This means that the two leading candidates, regardless of party affiliation, advance to the general election. This often results in two nominees from the same political party squaring off against each other.

Louisiana, for nearly a half century, has also preempted the party nomination process. In the Bayou state, there are no primaries except for presidential ones. There is just one general election. A runoff ensues only if the winner falls short of 50% of the vote.

Fourteen states, including New York and Florida, along with such battleground states as Pennsylvania and Nevada, have closed primaries, meaning voting is restricted to only those enrolled in the same political party as the party in whose primary they are voting.

Maine is one of three states – the swing states of Colorado and Arizona being the other two – deemed semi-open.

“Semi-open” means that a state allows voters, regardless of affiliation, to vote in any party they want subject to certain guardrails that limit their ability to do so. Though a non-party voter can vote in either primary, a member of an opposing party can only do so if they switch more than 15 days before the primary, a feature that keeps us from being completely “open.”

To be sure, the path to getting on a general election ballot is not necessarily through a primary (just ask Maine’s Independent U.S. Sen. Angus King) but it is still the predominant means of getting there – and a reason the latest change is worth a look.