The European Common Market is less common than it looks. With the rise of protectionism around the world, some of the more imaginative impediments to trade are appearing on the borders among the Market's 10 countries. It's particularly striking because no part of the world has seen more evidence of the ability of trade to lift national prosperity.

When the Common Market was founded 25 years ago, it agreed to abolish, gradually, all internal tariffs. That process ended in 1968. But that leaves very much intact what the experts call the non-tariff barriers. All the mechanisms of border control remain in force throughout Western Europe. The long campaign toward uniform customs documents within the Market still seems to be a long way from success. Lately there has been a good deal of amusement, except among the Dutch brewers, over the Germans' refusal to let into their country any beer that does not fully meet their own highly restrictive beer purity law, which goes back to the 16th Century. It has been noted--by The Economist, repeatedly--that automobile prices are much higher in Britain than on the Continent. Conversely, video recorders are conspicuously cheaper in Britain.

Two years ago the European Commission had on file some 300 complaints of protectionism in violation of Market rules. Currently there are 770. Recession is having the same effect in Western Europe as in most other places.

When the Common Market was founded, one reason was to duplicate the enormous and obvious advantage that the United States possessed in its own huge internal market. The Europeans made extraordinary gains for a time, and yet now progress seems to have slowed sharply.

The two economies are very similar. The combined population of the Common Market is slightly larger than that of the United States. But its economic output--GNP, measured in purchasing power--is slightly lower, about five-sixths the American level. There's no obvious reason why the Common Market shouldn't be fully as productive as the United States --no reason except the traditions of internal division, which have proved to be unexpectedly durable.

Perhaps that's not entirely unintentional. In Europe's politics over the past 15 years there have been repeated ripples of rebellion against fast economic growth. Everyone wants to be richer, but not many people really want to live drastically differently. Unfortunately, you don't often get one without the other. The architects of the Common Market always felt that if the European movement couldn't maintain momentum toward greater unity, it would slide rapidly backward. That risk remains as great in the 1980s as it was in the 1950s.