As the US heads into "mini Super Tuesday" - the second most significant day of the US primary elections so far - here is what you need to know and what is at stake.
:: What is mini Super Tuesday?
It is the second busiest day so far of the US primary elections. Also known as Mega Tuesday, it follows in the footsteps of Super Tuesday (March 1), and should not be confused with Super Tuesday Two (March 8).
:: Which states will be holding primaries?
Florida, Illinois, Missouri, North Carolina and Ohio. Plus one territory - Northern Mariana Islands - which is holding a Republican primary.
:: Why is it significant?
Three reasons. With more than one thousand delegates up for grabs (second only to Super Tuesday) it is a big day for both sides.
Some of the areas holding primaries on are known as "swing states" - similar to marginal seats in constituencies in the UK parliamentary elections. This means the result could go either way. Swing results are hard to predict because there is usually no consistent pattern to base projections on.
It is also the first day where the Republican party will not have to award delegates on a proportional basis rather than the winner-takes-all method. The anti-Donald Trump camp are hoping this will allow alternative Republican candidates to make up lost ground and catch up with the controversial front-runner.
:: How many delegates are needed?
A Democratic candidate must secure at least 2,382 out of 4,763 delegates to become the party's nominee. Meanwhile, a Republican candidate must secure at least 1,237 out of 2,472 delegates to win the party's nomination.
Donald Trump has 460 delegates so far, Ted Cruz 370 and Marco Rubio, 163. On the Democrats' side, Hillary Clinton has at least 1,231 delegates and Bernie Sanders 576.
:: Where could it leave the presidential hopefuls?
If Trump manages to win Ohio and Florida, experts have suggested it would be unlikely that a rival Republican candidate would gain more delegates than him. But Rubio and John Kasich are hoping to challenge him. A win for Rubio in his home state of Florida would raise questions about Trump's strength, as could a win for Kasich, Ohio's governor, on his home turf.
Hillary Clinton has a comfortable lead over her Democrat rival Bernie Sanders, but must deliver on Tuesday.
:: What happens next?
In July, Philadelphia will host the 47th Democratic National Convention and Cleveland will host the 41st Republican National Convention.
Some national conventions can be ceremonial - simply confirming that the leading candidate will be the party's presidential nominee. In other cases, where there is no clear front-runner, rounds of voting are needed to establish the party's nominee.
Once the Republican and Democratic nominees are decided upon, there is an election and their fate nominally lies in the hands of the 538 electors who make up the Electoral College.
This is because when the public choose a particular candidate to back, they are actually voting for their state's electors, who act as intermediaries.
Once a candidate has a majority in a particular state that state's electors can vote for that candidate.
A candidate must win the votes of at least 270 electors - half of the total plus one - to become president.