There are 28 states where marijuana is legal for medical or recreational purposes — but only two, California and Oregon, allow residents who were convicted of certain marijuana-related offenses to apply to have their sentences reduced or their criminal records expunged.

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When California voters approved a ballot measure to legalize recreational marijuana this month, they also made current and former marijuana offenders eligible for resentencing or expungement if they were charged for a crime that's now legal or carries a lesser penalty. Oregon passed a similar law through the state legislature last year.

The Orange County Register has a useful report that shows the difference in marijuana-related penalties under California's new law.

For example, growing less than six marijuana plants used to be punishable by up to two to three years in prison; now it's legal. Transporting up to an ounce of marijuana used to be a misdemeanor; that too is now legal.

Many states, including several that haven't legalized cannabis in any form, have already decriminalized marijuana — reducing the penalties for possession to about what you'd receive for a traffic violation. But minimizing penalties for marijuana offenses only goes so far; in California, where marijuana was decriminalized in 1975, there were still more than 500,000 marijuana-related arrests between 2006 and 2015, according to the Drug Policy Alliance.

Having a drug offense on your record can make it more difficult to find a job and rent a home — among other things — so reform advocates pushed to include the resentencing provision in California's legalization measure.

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Joy Haviland, a staff attorney specializing in marijuana law at the Drug Policy Alliance, told ATTN: that California's approach to past offenses should serve as a model for states that choose to legalize marijuana in the future.

"If the public determined that marijuana use should be legal because prohibition is more dangerous than marijuana itself, then it's in the interest of fairness for people that have these prior convictions — that have been previously punished for behavior that is now considered legal or less serious — that they should be treated similarly under today's standards," Haviland said.