Golf used to be a game of feel, and not just around the green. "This looks like a strong 6-iron," the pro would say to his caddie (or vice versa). Elevation, wind, temperature, green contours, and even mood could change a club selection.
Years later, caddies and pros got wise and began measuring yardages (often with yardage wheels like the one shown at right). Caddies would show up on Monday and Tuesday to walk the course, measuring every conceivable yardage and writing them all down in a little booklet. They'd include numbers that indicated the carry distance of a water hazard, draw arrows indicating severe slopes and funnels on the greens, predominant wind conditions, and notes about areas in which you did not want to miss.
In the 1990s, this too faded away as caddies with yardage wheels were replaced by ultra-accurate, GPS- and laser-measured yardage guides. Every pro has the same yardage book these days, tailored slightly to suit a particular pro's playing habits, and caddies spend their Mondays and Tuesdays without a yardage wheel in their hands.