Once unthinkable, tuition-free college has become a reality.

Four states and one city have enacted measures in the past three years. And lawmakers in several other places across the country are considering similar programs.

More than 30,000 Tennesseans and 7,000 Oregonians have gone to community college tuition free already. Students in New York and San Francisco are set to start on the same path this fall. And it's picking up steam, with lawmakers in several other places across the country considering similar programs.

The issue started drawing attention in 2015, when President Obama proposed making community college free nationwide. At the time, the idea sounded far-fetched to many -- but Tennessee had already approved the Tennessee Promise scholarship, which made community college free for students graduating high school that year. (The state is now expanding the program to all adults.)

The idea generated even more buzz during the 2016 presidential election when both Bernie Sanders and Hillary Clinton threw in their support. Obama, Sanders and Clinton wanted both the federal and state governments to split the tab. But the idea has not gained support from Republicans on the federal level.

States -- both blue and red -- have forged ahead, though, touting free tuition as a way to create a strong workforce.

Related: Paying for college without a scholarship

How the plan works

In most states, it's given in the form of scholarships that cover the remaining cost of tuition after using other needs-based grants. Some plans have an income cap and others are limited to recent high school graduates.

Free tuition doesn't mean there aren't any costs to the student. Students who live on campus also need to pay for room and board themselves. Sometimes there are additional fees that colleges charge for technology use, orientation, or other items, that aren't covered.

And yes, somebody's got to pay for it. Taxpayers are on the hook in most states, but Tennessee's program is fully funded by the state lottery.

Here's where tuition-free college stands nationwide:

Rhode Island

Recent high school grads who enroll at Rhode Island Community College don't have to pay anything for tuition or fees starting in the fall of 2017. They must maintain a 2.5 GPA in college while remaining enrolled full-time, and are required to live, work, or continue their education in state after finishing their degree.

The Promise Scholarship was approved as a four-year pilot program and is expected to cost $2.8 million in its first year.

A broader proposal from Governor Gina Raimondo failed to pass. It would have made two years at four-year colleges tuition-free as well.

Tennessee

Starting in 2018, all students in Tennessee including adults will become eligible for free tuition at the state's community colleges and technical schools as long as they don't already have an associate's or bachelor's degree. It is an expansion of a program that began in 2015 offering free tuition to students who had graduated high school the previous spring.

Students must be state residents for at least a year before applying. To keep the scholarship, they have to enroll at least part-time, maintain a 2.0 GPA and complete eight hours of community service each semester.

The program cost the state lottery fund about $12 million in the first year and is expected to cost an additional $10 million a year to include adults.

Related: Tennessee makes community college free for all adults

San Francisco

All 28,000 students at City College of San Francisco won't have to pay for their tuition, starting in the fall of 2017. The program is one of the most progressive because every resident is eligible no matter when they finished high school. And unlike other plans, it doesn't matter whether you're pursuing a degree or simply want to take one class. It also offers the poorest students additional money to help pay for these other expenses.

The city is increasing a real estate transfer tax on luxury properties to pay for the scholarship, which is expected to cost $5.4 million over the first two years.

New York

In April, New York became the first state to make tuition free for both two- and four- year colleges beginning this fall. Eligible undergraduate students won't have to pay anything for tuition at a State University or City University of New York school.

But students whose families earn more than $125,000 a year won't be eligible. Even though you don't need to be a recent high school grad, you cannot already have a degree. You also must enroll as a full-time student and are required to live and work in New York for the same number of years you received the scholarship.

The program is expected to cost the state $163 million a year.

Related: A degree from this college all but guarantees you a job

Oregon

Students who started community college in the fall of 2016 were the first to benefit from the Oregon Promise scholarship, which covers most of tuition for recent high school graduates and GED recipients. Adults returning to school are not eligible.

Students must be a state resident for at least a year before applying, earned a minimum of 2.5 GPA in high school, and enroll at least part time.

The program cost the state $10.9 million during the first year.

Because of a state budget shortfall, the state was forced to limit eligibility for the 2017-2018 school year. Starting this fall, students from high-income families will be excluded.

Related: What you need to know about New York's free tuition

Arkansas, Minnesota, South Dakota focus on high-demand areas

In Arkansas, a new grant will make tuition free for certain students at community colleges and technical schools starting this fall. Students must be enrolled in a high-demand field of study, such as computer science or welding. It's similar to programs in South Dakota and Minnesota that makes tuition free for students studying in fields where there is a high demand for workers.

Louisiana

Louisiana's Taylor Opportunity Program has covered tuition for students who meet certain academic standards for decades. It covers the entire cost of tuition as long as students graduated from high school in-state and met two academic requirements: a 2.5 high school GPA in core classes and at least an average standardized test score.

But, for the first time, the cash-strapped state could not afford to fund the scholarships fully for the 2016-2017, leaving some students scrambling. Funding has been restored for the upcoming school year.