Golf courses have 18 holes because of a round played at St Andrews in Scotland in 1764.

The Old Course at St Andrews originally had 22 holes, and golfers played all of them out and then back in again for a round of 22. In 1764, the club’s members decided that several of the holes were too short, so they combined some of them, which reduced the course to 18 holes.

Because St Andrews was one of the most influential clubs in the sport’s early development, other courses gradually adopted the same 18-hole layout as the standard, and by the 20th century it had become the accepted norm for golf courses worldwide. The number has no particular technical or competitive rationale behind it; it’s essentially a historical accident from one Scottish club’s 18th-century course redesign that the rest of the golfing world simply followed.