The golfer looks at his ball, then turns to his caddie with eyebrows arched in question. "See that big, high grass over there?" answers caddie Jack Hopkins. "You have to be 20 yards to the right of it." The golfer nods his thanks and swings.

Thwack! The ball soars off, an arcing, vanishing white dot in an immense blue sky; it lands on the green. The golfer nods his thanks again.

Minutes later, another golfer in the same Merion Golf Club foursome lines up a putt, then turns to caddie Pancho Thornton. "What do you think?" Without a nanosecond of hesitation, Thornton says: "Aim at the right edge. The ball will break to the left." He does, it does, and the ball drops into the hole.

Out of the golfers' hearing, Thornton whispers, "Three rules of caddying: Show up, keep up, and shut up unless somebody asks."

In addition to carrying 20-pound golf bags on their shoulders, Merion caddies carry something special in their heads: The Knowledge. Golf, like war, is most often a question of terrain - and no one knows the lay of the land at Merion's 97-year-old East Course in Ardmore like its caddies.

"Nine months out of the year, we work 36 holes nearly every day," says Hopkins, who began caddying here 31 years ago. "You get to know it pretty well." His eyes crinkle sagely.

But it is The Knowledge that will place an elite corps of Merion caddies under considerable pressure during the two-day Walker Cup Match, which will begin Saturday and pit 10 top American amateurs against a 10-man team from Britain and Ireland.

"The pressure will be on the caddies as never before," says Scott Nye, Merion's head pro. "For most of them, this is the biggest tournament they've ever worked. Unlike professional tournaments, where the golfers bring their own caddies, the Walker players will be using local caddies because they know the course so well. Our caddies will be making decisions that could directly affect the outcome of the play."

Thornton, who has been caddying at Merion for 25 years, agrees with a rueful grin. "To one degree or another, the player is going to have to trust me. In all modesty, very few people know this course as well as me."

Each of the 20 Walker Cup participants will use a caddie from a pool that was culled - based on performance during the last year - from the core group of 40 Merion caddies.

All are independent contractors, and they deal with each golfer separately. The average fee for 18 holes, including tip, is $65 to $75 per bag. For the Walker Cup, they will be paid a flat $800, which includes practice sessions, a practice round Friday, and competition Saturday and next Sunday.

For each round, player and caddie will walk about five miles over Merion's fabled East Course, where not only motorized carts are banned but also electronic distance finders, GPS locators, and other gadgets that are now part of most courses. Moreover, on other courses every sprinkler head carries the distance to the next green; at Merion, there are no yardage markers anywhere. That information rests exclusively with the caddies, who are expected to know how far it is to the green from any point on the 126-acre course.

Bob Morey, who oversees the caddie program as chairman of Merion's Golf Committee, says caddie expertise will be most critical on the greens.

"Our greens are diabolical," he says. "There are slopes, up and down, and the way the grain of the grass grows influences how the ball will break. No one knows the idiosyncrasies of a green like a man who has studied that patch of grass almost every day for 25 years or so."

Arnold Palmer said this about caddies: "You're a golfer, or you're a person who plays golf. To me, people who walk and use caddies are so much more in tune with the game." Despite his words, the caddie is an endangered species in America. According to Sportometrics, a South Carolina golf research firm, only about 7 percent of all U.S. courses use caddies, let alone require them.

Yet caddies are almost as old as golf itself. As the game became more complex from its humble beginnings 600 years ago, the single club evolved into an assortment of specialized clubs. These required a bag, and the bag required someone other than the player to carry it.

In the early 19th century, nearly every golfer had two caddies - one to carry the clubs, the other to station himself down the fairway to keep track of balls, which were very expensive.

Most golf historians date the invention of the golf cart to around 1940, and it was then that the decline of the caddie began. Today the carts, which come in electric and gas versions, are everywhere and getting more elaborate. GPS systems are built in to gauge distances, and it's only a matter of time before wind speed, wind direction, elevation changes, and the other bits of information once only inside a caddie's head will be available on a cart's computer. Carts also enable players to carry backpacks with rain gear, offer on-screen professional tips, and take lunch orders that can be picked up at the next tee.

"I haven't seen yet a golf cart that can read a green or find a ball in the rough," snorts Thornton. "The cart can't select a club, but it might run it over."

Morey concurs. "The way that golf is really played is with a caddie. This is how it was meant to be played. Period."

Nye notes that the form of the Walker Cup Match will intensify the caddies' task.

"Part of the Walker Cup play is an alternate-shot competition where two players and two caddies will be involved on the same team," he says. "The two caddies, as well as the two players, will have to interact. Things will really get complicated."