Recreational marijuana will soon be legal across the Mississippi River in Illinois.

On Tuesday, Illinois Gov. JB Pritzker signed a bill legalizing the possession and use of marijuana, and expunging the criminal records of people with minor pot possession convictions.

That means adults ages 21 and older — including Iowans visiting the "Land of Lincoln" — will be able to purchase marijuana from licensed sellers and use it while in the state starting Jan. 1, 2020.

Illinois joins 10 other states that have legalized recreational use of marijuana: Alaska, California, Colorado, Maine, Massachusetts, Michigan, Oregon, Nevada, Vermont and Washington.

Recreational marijuana use remains illegal in Iowa.

Here's what Iowans should know about the new Illinois law and what it might mean for you:

What does the law allow?

Illinois residents will be able to purchase up 1 ounce of raw cannabis as well as cannabis-infused products, such as edibles or tinctures, that contain no more than 500 milligrams of THC, the chemical that makes users high.

Can Iowans buy marijuana in Illinois?

Non-Illinois residents, including Iowans, can purchase and possess half the amount residents are allowed to possess.

Just don't bring it back across state lines.

The possession and sale of marijuana are still illegal at the federal level as well as in Iowa. Selling marijuana in the state is currently a felony, punishable by up to 50 years in prison.

Will there be marijuana dispensaries in Illinois?

Yes. Licensed dispensaries will be the only place where you can legally buy recreational marijuana if the law goes into effect.

Illinois, which began selling medical marijuana in 2015, now has 55 such dispensaries. Those dispensaries can apply to sell pot for recreational use, too, and the new law also allows them to open a second location for that purpose.

According to The Chicago Tribune, some dispensaries are already planning building expansions, staffing increases and new technology to handle the recreational marijuana customers and an expected increase in medical marijuana sales.

Where could you legally use recreational weed in Illinois?

Under the new law, people will be able to smoke or consume edibles in their homes, as well as in certain cannabis-related businesses.

However, marijuana use will still be prohibited in these places:

Any public place, including streets or parks.

In motor vehicles.

Near someone under 21.

On school grounds, except for medical marijuana users.

Its use also could be prohibited on private properties, at the owners' discretion.

What's the marijuana situation in Iowa?

While recreational marijuana is banned, Iowa allows patients to use cannabidiol for certain medical conditions, including epilepsy, cancer, multiple sclerosis, Crohn’s and Parkinson’s disease.

The medical cannabidiol act was first passed in 2014 and updated in 2017 to include a manufacturing and dispensary system.

This spring, the Iowa Legislature passed a bill that would have further expanded the program, removing a 3% cap on THC in medical marijuana products.

However, the legislation was vetoed by Iowa Gov. Kim Reynolds last month.

“Ultimately, I believe Iowa must proceed cautiously to ensure that any expansion of our medical (cannabidiol) program is thoughtful and deliberate," Reynolds said.

What does the federal government say about marijuana?

Marijuana remains illegal at the federal level, but since 2014, federal prosecutors have generally ignored marijuana sales in the states that have legalized its use.

Reporters Kellie Hwang and Andrew Clark of IndyStar and Trevor Hughes of USA TODAY contributed to this story.

Editor's note: This story has been updated. Doctors do not prescribe cannabidiol under Iowa law.