The USGA has periodically issued guidlines for the par-ratings of holes based on their lengths; i.e., if a hole is 200 yards, it is a par-3.
Those guidelines have changed over the years, and the way they are used has changed, too.
The current guidelines are these for men:
Par 3 - Up to 250 yards
Par 4 - 251 to 470 yards
Par 5 - 471 yards to 690 yards
Par 6 - 691 yards or more
And for women:
Par 3 - Up to 210 yards
Par 4 - 211 to 400 yards
Par 5 - 401 to 575 yards
Par 6 - 575 yards or more
It's important to note that the current USGA guidelines are not, in fact, based on actual yards, but on a hole's "effective playing length." Effective playing length is one of the factors taken into account when a course is given its USGA Course Rating and USGA Slope Rating.
Effective playing length, and why it's used in these guidelines, is pretty easy to understand. Say a hole's actual yardage is 508 yards. That hole, by its actual yards, might be a par-5. But what if the hole plays downhill all the way? It's effective playing length - how long the hole actually plays like - might be closer to 450 yards. Therefore, a hole whose actual length might make some think it should be par-5 really only plays like a par-4. According to current guidelines, that hole would be a par-4 (the guidelines are not hard and fast rules, by the way, but simply ... well, guidelines).
Prior to the introduction of effective playing length into course ratings, the guidelines were based on actual yards. It's interesting to see how they've changed over the years. We have three examples below; in each case, the yardages listed are for men:
1911
Par 3 - Up to 225 yards
Par 4 - 225 to 425 yards
Par 5 - 426 to 600 yards
Par 6 - 601 yards or more
1917
Par 3 - Up to 250 yards
Par 4 - 251 to 445 yards
Par 5 - 446 to 600 yards
Par 6 - 601 yards or more
1956
Par 3 - Up to 250 yards
Par 4 - 251 to 470 yards
Par 5 - 471 yards or more
