As per Sean's post: when golf moved in-land from the links, and the great old architects in the UK finally figured out how to make it work. It is the most important development in golf, if for no other reason than because there is a heck of a lot more "in land" available than "links land". I'm a little sad to see that, still to this day, it is the sand-based sea-side courses (new and old) that still get an inordinate amount of attention and praise; while to me the real marvels are the quieter, plainer inland courses that have managed to serve their purpose for a hundred years.
Peter