Labor day isn't just an excuse for millions of workers to have a three-day weekend. It began as a union holiday, an American counterpart to the International Workers Day of May 1st. But while the holiday endures, unions are increasingly becoming a thing of the past in the US. Here's a chart-filled rundown of how unions' place in the US has fallen off over the years, and what that means.

1) Unions have shrunk — a lot.

Just 30 years ago, around 1 in 5 workers was a union member. Today, it's just over 1 in 10, around 11.3 percent as of 2013. The cause of the decline is subject to heated debate. One reason may be new right-to-work laws — five states have added right-to-work laws since 1980. Some have argued that unions simply don't appeal to young workers like they once did.

2) The fall happened entirely in the private sector.

Today, there are more than 3 million fewer union members than there were in 1983. But public sector unions have still grown; only private sector unions have fallen off, by around 4.6 million people.

3) The number of "right-to-work" states has slowly grown.

Unions in some states can require workers to pay dues to a union despite not being a member of that union. The rationale is that non-union employees at a union workplace are free riders, getting the benefits of that union without paying into it. In "right-to-work" states, forcing employees to pay dues is illegal. Today, there are 24 states with right-to-work laws on the books, the majority of which were passed in the 1940s and 1950s, according to the National Conference of State Legislatures. That was right after Congress passed the Taft-Hartley Act in 1947, which allowed states to enact right-to-work laws.

4) While unions have shrunk, inequality has grown. That may not be a coincidence.

Deunionization in the US has occurred alongside rising inequality, and some economists have suggested a causal link between the two phenomena.

One 2011 paper from researchers at Harvard and the University of Washington estimated that "the decline of organized labor explains a fifth to a third of the growth in inequality" in hourly wages. Another 2003 paper from researchers at the University of California-Berkeley and the University of British Columbia found similarly that de-unionization explains growth in wage inequality, but only for men.

Unions can have all sorts of effects on the labor market, of course, and not all of them positive. Unions contribute to unemployment, as Larry Summers has written, by helping set higher wage floors (though he is quick to point out that unemployment was a problem long before widespread unionization).

5) Unions are way bigger in other countries

Unions are far less prevalent in the US than they are in almost every other highly developed nation. Among the 23 OECD nations for which there is data, 20 had higher unionization rates in 2011. These higher levels of unionization may be one reason why many of these nations also have higher levels of paid leave and vacation time than the US.

6) Unions have declined in many places beside the US.

It's true that other nations have far higher unionization levels than the US, but unionization has fallen worldwide. Since 1999, unionization in OECD nations has declined from around 21 percent to 17 percent, decreasing or staying constant in nearly every OECD country.

7) Public support has held firm.

Even while unionization rates have fallen off, Americans' support of unions has stayed relatively steady. According to data from the Pew Research Center, around half of Americans have viewed unions favorably since the mid-1980s. And unions remain a formidable force in politics; labor accounted for nearly 14 percent, or $115.6 million, of all outside spending in the 2012 election, according to the Center for Responsive Politics. However, there is conflicting data on this. Gallup has found that the share of Americans who say they approve of unions has slowly tapered off since the 1950s.