Did you know that you may have to vote again in your primary if no candidate gets more than half the votes? If not, you may want to find out if this applies to you.

For example, there’s been such a close race for Jeff Sessions’ vacant Senate seat in Alabama that the Republican primary in August did not determine a winner. A primary runoff between the top two Republican candidates, Roy Moore and Luther Strange, is scheduled for September 26.

In over 115 races that Vote Smart has already tracked in 2017, we have encountered other unconventional election practices that voters may not be aware of. Earlier this month, Mississippi held a nonpartisan primary election for its State House Dist. 102 seat that will be determined by a runoff election in October.

Most Americans participate in a first-past-the-post system where winners in primaries and general elections are decided by a “plurality”—the candidate with the most votes wins. Several cities and states across the nation, however, have adopted alternative election methods. Read on for a brief explanation of the unique election practices that are currently used at the state and federal levels.