18 Hole Round
Many people have asked why golf courses have eighteen holes.
The early golf courses all had different numbers of holes.
Leith Links had 5 holes in 1744 and added 2 holes later.
Blackheath followed Leith in having 5 holes and expanding to 7 holes.
Bruntsfield Links also had 5 holes, but could only expand to 6 holes in 1818.
Musselburgh Old Course had 7 holes for many years, added an 8th in 1832 and a 9th in 1870.
Montose Links had 7 holes by 1810; 14 holes by 1825; 11 holes by 1849; and 25 holes by 1866, though these were reduced sometime shortly after 1874.
St Andrews (Old Course) had 12 holes by 1764, and probably much earlier. The holes were laid out in a line and 10 holes were played twice, once 'out' and once back 'in', making a 'round' of 22 holes. In 1764, the golfers decided to combine the first four holes into two, which were short holes to produce a round of 18 holes, though it was really 10 holes of which 8 were played twice.
Therefore, when Prestwick was built in 1851 with only 12 holes, it did not look out of place.
Although some clubs were playing 18 holes as medal round at this time, it was purely be accident, such as at Lanark from 1851-1853 where they had six holes that were played three times. When they added another hole it became a round of 21 holes.
There is evidence of two holes on one middle green on the Old Course in 1821 and there may have been on others, but they may have used alternately to spread the wear on the green which also served as the tee'ing area at this time.
From 1842, the Royal and Ancient was playing 18 holes in competitions and it is believed that two holes were being used on the middle greens at the Spring Meeting of May 1857, as reported in the Fifeshire Journal.
The course was played in the clockwise direction in this period, though shortly after it was alternately played in clockwise and anticlockwise directions to manage the wear of the course.
Other courses began adopting this standard over the next decades, as detailed in Oldest 18-hole Courses. The main driver behind this appears to have been the influence of prominent members of the Royal and Ancient Golf Club who were members elsewhere, though it took 25 years before there were 18 golf courses in the world with 18 holes, as detailed in the Oldest Courses.
The second course with 18 holes was Montrose, which developed dozens of courses over four different playing areas over the centuries. By 1863 they had 25 holes and the Royal Montrose were playing their medals over 18 holes, though it would not be until 1888 that they had a defined course of 18 holes.
The third course with 18 holes was at Dubbieside in Fife in 1866, but the Innerleven Golf Club decided to abandon it and move to Leven golf course, which was extended to 18 holes to Lundin in 1868, and this becomes the third extant 18 hole golf course.
From 1872, the British Open golf championship was held annually in rotation at Prestwick, St Andrews and Musselburgh, where the three sponsor clubs were based. The contest was over 36 holes and it was, therefore, three rounds when it was held at Prestwick, two rounds when at St Andrews and four rounds at Musselburgh. The competition must have created comparison of the courses and the 18 holes at St Andrews would have seemed the most appropriate.
Thus, in 1882, Prestwick expanded its course to 18 holes, the 18th course to do so, and in 1891 when the Honourable Company built Muirfield they created 18 holes in the first year. As they sponsored the Open, the championship moved with them from Musselburgh to Muirfield. With the three foremost clubs in the world using 18 holes, this set the norm for a golf round.
The 18 hole round was a default found for a golf match from 1933, but it was not laid down as a 'stipulated round' in the Rules of Golf until 1950. As late as 1919, when the Royal and Ancient took over sole control of running the Open, half of all the golf courses in Britain were still built as 9-hole courses.
Therefore the reason why golf courses are 18 holes is partly at least an accident of history.
More detail of the development of the Old Course at St Andrews.