ONE LATE winter morning, so the story goes, Charlemagne was standing in his snow-surrounded palace at Ingelheim, on the left bank of the Rhine. As he looked across the river, the warrior-king noticed that the snow was already melting on the far bank. Recognizing the potential of the sunny slopes and using the unlimited powers of a Holy Roman emperor, he ordered that grapevines be planted on the right bank, immediately. In time, the vineyards flourished and the region became known as the Rheingau, considered by many to produce Germany's most prestigious wines.

In the center of this narrow strip along the Rhine are the towns of Erbach and Hattenheim, and here are the vineyards of Schloss Reinhartshausen. From Charlemagne, in the eighth century, down to the present, the ownership of this private estate has been held in noble hands.

Prince Nicholas von Preussen, great-grandson of Kaiser Wilhelm II and the present owner, came to Washington recently. At the invitation of the German Wine Society, he was on a 10-city 17-day tour of North America to promote the wines of Schloss Reinhartshausen. Accompanied by his export manager, Tim Jackson, Prince Nicholas had star billing at the tasting of his wines organized by the Washington chapter of the society.

His visit here coincided with the annual general meeting of the German Wine Society, a nationwide association of wine professionals and amateurs. And the Washington chapter had both the pleasure of introducing a bonafide wine-producing prince and announcing that its immediate past chairman, Bob Baughman, had been elected president of the national society.

In a division of responsibilities, which could be labeled the Prince and the Pourer, it was Tim Jackson who provided the commentary for the tasting held at the German Embassy. The Reinhartshausen estate owns some 10 vineyard properties, producing 30 different wines each year. Most are made from Germany's most noble grape, the riesling. Other varieties grown include weissburgunder (pinot blanc), traminer and kerner.

However noble the grapes, their lineage is overshadowed by that of the long line of owners of the estate.

There was Princess Marianne of the Netherlands, who bought the estate in 1855, because, we're told, she admired the beautiful countryside -- and, we're sure, the wines it produced. The princess, who was obviously an intelligent woman, then married into the Hohenzollerns, the royal family of Prussia. Her husband, Prince Albrecht von Preussen, was a brother of the Wilhelm who later became the first kaiser of Germany. Since Princess Marianne's purchase and her subsequent marriage, Schloss Reinhartshausen has remained in the von Preussen family.

Today, half of the entire production of this historic estate is vinified as trocken and halbtrocken wines, the dry styles that have been the subject of much controversy. The principle behind the trockens is to reduce the residual sugar content of the normally fruity German wines. However, so far they've only achieved popularity in Germany itself. The modern German is diet conscious and is looking for a drier white wine to drink with food, rather than the traditional occasionally sweet whites that are better without food.

In export markets, lovers of German wines have still to be convinced that the reduced calories of trockens are worth the loss of the intrinsic fragrance and fruit that are the hallmarks of German-style wines. Schloss Reinhartshausen, aware of our skepticism, does not export its trockens.

However, to show us just what improvements are being made on this comparatively new type of wine, Tim Jackson presented three recent examples: an '80 Erbacher Rheinhell Weissburgunder Kabinett trocken, a '79 Erbacher Steinmorgen Riesling halbtrocken, and a '79 Erbacher Siegelsberg Riesling Kabinett halbtrocken. As far as trockens go, this trio wasn't bad, or at least, no more characterless or flavorless than any others.

To the relief and pleasure of participants, the remaining wines in the Reinhartshausen tasting were more typical of the style and quality of a top Rheingau estate. A comparison of the '79 and '71 vintages of the Hattenheimer Wisselbrunnen Riesling Spatlese showed the younger to be lightly fruity, while the older was fuller, smoother and altogether more interesting, as expected of the fine 1971s.

Two '76 riesling auslesen followed. The Erbacher Schlossberg had an edelfaule, or botrytis, edge. It was golden in color and had a delicious bouquet of peaches and honey. The Marcobrunn, the top-ranked vineyard of Erbach, was earthier, tasting of apricots.

The evening's dessert was a '71 Erbacher Siegelsberg Riesling Beerenauslese: a rich wine, showing its maturity in its deep tobacco-gold color and honeyed nose.

German wines have struggled to hold onto their share of the American market in recent years. Currently accounting for 13 percent of the imports on American shelves, the demand for German wines has been undermined by two factors: the unexciting 1977 and 1978 vintages plus the heavy promotion given to the drier white wines of Italy and France. However, the pleasant '79s and the stronger dollar have made the better German wines an attractive buy.

Tim Jackson was happy to admit that his visit to the United States was more than just good will. "We are convinced that there is a very knowledgeable and open market here for quality German wines, and, in view of the uncertain international economic situation, we'd like to expand our distribution in the United States."

Schloss Reinhartshausen is a member of the 162-strong Association of German Pradikat Wine Estates, which sponsors an annual auction in Mainz. At last year's auction, a 1911 Erbacher Markobrunn Trockenbeerenauslese from Reinhartshausen was sold for $5,500. Described as "crisp and alert with noble fruit and botrytis, elegant with a peachy aftertaste," the TBA was eclipsed in price by two bottles of a 1648 Johannisberger Rheingau, which were sold for $8,500 and $7,500. But, the purchaser of the Markobrunn could have the last word. Jackson reported that the 1911 was still in excellent drinking condition, a testament to the storage at the Schloss and the fine quality of the soil and grapes.

Charlemagne knew a good thing when he saw it.