Most golfers know a full round of golf is 18 holes. Half of a round of golf is nine holes. But why does a golf course have 18 holes? Why not 20? Or 10? Or some other number?
The reason why most golf courses have 18 holes is because we're all copycats of the Home of Golf.
Why golf courses have 18 holes
Golf courses have 18 holes because of the example set by the Old Course at St. Andrews in Scotland. While it's not the first ever golf course built, the Old Course is considered the Home of Golf, the most important course to the history of the game.
St. Andrews' Old Course didn't always have just 18 holes. In fact, early in its history, the Old Course, which wasn't always called that, actually had 22 holes. At the time, there was no standard for how long a golf course should be or how many holes it should have. There was no such thing as standard round. Some courses had 12 holes, others 20, and there was no fixed number or way to compare golf scores from one course to the next.
That was still true when St. Andrews ultimately made the choice to turn the Old Course into an 18-hole course in 1764, making two longer holes from four shorter holes. There would be nine holes going out from the clubhouse to the farthest point on the property. Then golfers would turn around and play another nine holes toward the clubhouse. Hence the terms "out" and "in" or "out" and "back" on a golf scorecard because golfers literally went out and came back in when they were done.
Even still, the concept of an 18-hole golf course didn't take foot until much later. Prestwick Golf Club, which held the first dozen British Open Championships, opened in 1851 and had 12 holes. Tournaments had three rounds to get to 36 holes.
St. Andrews first hosted the Open in 1873, and they played two 18-hole rounds to determine a winner over 36 holes. It just so happened that 12 and 18 had an easy common multiple along with 1874 host Musselburgh, which had nine holes. Come 1881, Prestwick had added six holes to get to 18, and by then St. Andrews' 18-hole course had become the standard on which future golf courses were judged.
That's why golf courses are 18 holes and why most golf courses have separate nines, to go out and back.