Why Golf Courses Have Eighteen Holes

From A History of Golf by Louis T. Stanley:

A primitive lay-out, must have been in existence before 1413. The number of holes varied over the years. The original links at Leith and Blackheath had five holes, later extended to seven like North Berwick and the London Scottish Volunteers on Wimbledon Common. St Andrews settled for twelve, which eventually became twenty-two. The problem was the narrowness of the strip of land available. Being less than forty yards wide, it ruled out separate holes for going out and coming home. Golfers played eleven holes out to the turn by the Eden estuary and returned using the same fairway and greens.

The twenty-two-hole course began near the Martyr's Monument. In 1764 the Royal and Ancient Club passed a resolution that the first four holes should be converted into two, a change that reduced the round to eighteen. Eventually six of the nine greens were extended laterally to allow two holes to be cut upon them, making possible the enormous double greens for which the Old Course is famous. A new site for the seventeenth green was chosen. When the eighteen separate holes were first played, the original nine holes were used on the outward half and the six holes on the extended greens with the new seventeenth green on the return journey.

By 1842 the general lay-out of the links was as today. The outline of the course has not changed. Up to the First World War there were right-hand and left-hand courses used alternately a week at a time. By accident the 1886 Amateur Championship, won by Horace Hutchinson, was played on the left-hand course. According to the rota, the right-hand one should have been used. The Championship was under way before the officials realized what had happened, the left-hand course being used for the first and last time. Steeped in history and tradition, the old course is a priceless possession, the ultimate test of golf, a centuries-old shrine of golf.