The 2016 Republican primary is on course to become the most crowded in modern presidential history.

The GOP now has 17 major contenders for the nomination — former Virginia Gov. Jim Gilmore announced on Wednesday — breaking the record previously held by the 16 candidates who sought the Democratic Party’s presidential nomination in 1972.

Counting presidential contenders isn’t an exact science, however. There are hundreds of candidates each election cycle that run for president. And the term “major” is, of course, subjective.

But it’s clear that 2016 is shaping up to be a record-setting election cycle. To understand why, it’s important to know a little history about the presidential nominating process.