It must have seemed a nice, handy phrase at the time, to describe the supremacy of an extraordinary amateur. But we have learned from letters published only last month, that Bobby Jones took serious exception to the notion that he was "beating the pros at their own game."

Yet the truth is that he did it on a regular basis, though never to such effect as in 1930, when he claimed a unique place in golfing history. That was when he beat the best professionals of the day to capture the US Open and the British Open which, added to the US Amateur and British Amateur, completed the so-called Impregnable Quadrilateral. With absolute safety, it can be described as a feat which will never be equalled.

When writing to Charles Price, the founding editor of Golf magazine in August 1967, Jones insisted: "Actually, we were all playing the same game, and if I played less that they did, that should have been the concern of no one but me; presumably, I could have played more had I wanted to."

Since 1926, when he became the first player to win the US Open and the British Open in the same year, Jones had set his sights on the biggest prize of all. And he knew that if he were to achieve such a colossal objective as the four major national championships in the same year, he would need to play more than usual, to have his game in appropriate shape.

A measure of his self confidence is that it didn't seem to bother him who might be aware of this awesome target. Indeed, further on in that letter to Price he wrote: "I am not handicapped by modesty."

By way of illustrating the point, he referred to "the grace and power of my stroke", before adding: "I get a little miffed, I admit, at all the conversation about the distances obtained by modern players with their steel shafts. We have always had one or two men who were longer than the rest, but I do not believe that in actual yardage, the modern player is very much longer than some of us were when we wanted to be."

He concluded: "To me, the most flattering comment I could hear was that the observer could not believe that the ball could be driven so far with so little apparent effort."

Given the overall quality of his game, it seems remarkable that Jones played only four tournaments in 1927 and a total of only five in 1928 and 1929. So, it was hardly surprising that he felt his skills were in need of honing before the first Grand Slam event of 1930, the British Amateur at St Andrews.

As it happened, the build-up provided exactly the sort of boost Jones was seeking. In a thrilling climax to the Savannah Open, Horton Smith, then only 22, shot 278 to beat Jones by a stroke. But when they faced each other again in the South-eastern Open at the Augusta Country Club, Jones was in irrepressible form, shooting 284 to win by no fewer than 13 strokes.

Some years later, he confessed to a friend that he had never played better at any stage of his career. So, the omens were decidedly good.

Then came the Walker Cup on May 15th and 16th, when he played a captain's role in America's crushing, 10-2 victory at Royal St George's, Sandwich. And there would be special significance in his 9 and 8 singles win over the British number two, Roger Wethered.

In a decidedly bleak introduction to the Old Course in the 1921 British Open, Jones was so incensed by its idiosyncrasies that he was moved to tear up his scorecard during the third round, after taking five at the short 11th. By the start of the Amateur on Monday, May 26th 1930, however, he had grown to love everything about the Auld Grey Toon, which was reflected in the quality of his play.

For 143 holes of matchplay, he was six under fours and had the best card of any competitor in each of the championship's eight rounds. And he was a huge favourite with the locals, who dominated the attendance of 15,000 for the final - against Wethered.

This time, the margin for the 36-hole match was 7 and 6 in Jones's favour and when they shook hands on the 12th, the charismatic American needed protection from the Scottish police while fighting his way through the adoring crowd who treated him like royalty. Later that day, he confided to his biographer, O B Keeler, that if he were never to win another tournament, this was all he ever wanted.

Two weeks later, after he and his wife Mary had celebrated in Paris, Jones was at Hoylake for the British Open. By current standards, his aggregate of 291 after a final round of 75 was decidedly modest, but as he waited in the clubhouse, it was considered good enough to win.

This was when the strain, which would lead to his retirement later that year, had begun to show. With Leo Diegel and Macdonald Smith out on the course within reach of Jones's target, he ordered a whiskey and soda. Suddenly, his hands began to tremble so badly that he could hardly hold the glass.

We are told that he felt obliged to grip the glass in both hands and after gulping down the soothing spirit, he ordered another, looking anxiously out through the clubhouse window all the while. In the event, Diegel faded and Smith needed an eagle at the last to tie. And as Smith's approach shot rolled past the 18th pin, the tension within Jones eased sufficiently to allow him take his left hand from the glass.

Afterwards, he told reporters: "This is my last shot at the British Open. It's quite too thick for me. I feel I'm not strong enough to play another one."

On his return to New York in SS Europa, friends from Atlanta were there in a ferryboat to take the hero off, while fireboats sent fountains into the sky. With degrees in engineering, literature and law and the looks of a movie star, Jones was revered as the epitome of a southern gentleman, a long-overdue hero after the humiliation of the civil war. Even the Yankee mayor of New York, Jimmy Walker, was more than happy to accord him a repeat of his 1926 ticker-tape reception down Broadway.

The next day, Jones was on a train bound for Minneapolis, where he faced his third major assignment, on July 10th to 12th.

There were historic elements to the 1930 US Open at Interlachen, other than as the third leg of the Grand Slam. For instance, it was the first time the event was broadcast by radio, with CBS using a portable transmitter and microphone to report from the course. And it was also the first US Open in which the score of each player was announced after the ninth and 18th holes.

Jones, who was defending the title he won at Winged Foot the previous year, opened with a solid 71 in sweltering, 100-degree heat, to be a stroke behind Macdonald Smith and Tommy Armour. By the halfway stage, he was two strokes adrift of a new leader, Horton Smith, after a second round that included a remarkable birdie at the ninth.

Despite pushing his drive to the right at the 485-yard par five, Jones still attempted to carry the lake with his second shot. Then, just as he was about to strike the ball, he noticed two little girls darting out of the crowd up ahead. The distraction caused a topped shot which was on such a low trajectory that it seemed certain of a watery grave.

But miraculously, it made dry land. Skimming the water twice, like a flat stone, it eventually hopped up on the bank on the other side, before coming to rest 30 yards from the pin, from where Jones pitched and putted for a four. Some observers were convinced the bouncing ball had hit a lily-pad but the general consensus was that it had hit only water.

With a brilliant, four-under-par third round of 68, Jones took the lead by no fewer than five strokes from Harry Cooper, with Horton Smith and Johnny Golden a further stroke away. Nothing could stop him now and in the final round, a generous official seemed determined this should be so.

From a pushed tee-shot at the par-three 17th, Jones's ball couldn't be found. But referee Prescott Bush (grandfather of former US President George Bush), ruled that the ball had gone into a water hazard and permitted the player to take a penalty drop on the fairway. With the help of this liberal ruling, Jones made a double-bogey five.

But he finished with a birdie for a closing round of 75 and a one-under-par aggregate of 287, two strokes clear of Macdonald Smith, who collected the top professional prize of $1,000. In the clubhouse that evening, Jones vowed to himself that he would retire, irrespective of what happened in the forthcoming US Amateur.

In his book The US Open: Golf's Ultimate Challenge, Robert Sommers added a beautiful postscript to the events at Interlachen. He wrote: "Four thousand miles to the east, Bernard Darwin rose early to read the reports from Minneapolis, because, like most Britons, he was pulling for Jones. Seeing that Bobby had won, he went for a walk and along the way he met J H Taylor (winner of five British Opens).

" `So Bobby has done it again,' Darwin said. `Wonderful, sir, wonderful,' Taylor said. `I can't understand how Mr Jones can keep it up.' "

With more than two months to go to the US Amateur at Merion, Pennsylvania, the strain had now become enormous. But Jones knew there was no turning back. Quite apart from the expectations of the golfing world, he owed it to himself to confront the final hurdle.

So intense was public interest that one US newspaper alone sent a team of 16 writers to cover the event while 50 marines were assigned to protect Jones on the fairways of Merion. As it happened, the final challenge was somewhat anticlimactic, certainly from a competitive standpoint.

Jones won all his matches comfortably, including the final in which Gene Homand was disposed of by 8 and 7. The impossible had been achieved; the Grand Slam was his.

On November 17th, 1930, Bobby Jones announced his retirement from competitive golf. At only 28, he had achieved the staggering record of 13 triumphs in 21 national major championships. In the 11 years starting in 1920, when he made his US Open debut, he won seven of the 15 US and British Opens in which he competed.

Almost 70 years on, it is a record which remains unmatched. All of which lends telling emphasis to the words of the celebrated US golf writer Herbert Warren Wind, who wrote of Jones's retirement: "There were no worlds left for him to conquer."