THOUGH BRUSSELS MAY NOT have gotten the word, a new "common market" of a sort has been created right here in our midst. It is called the Baltimore/Washington Common Market - and it is a most sensible and important idea that springs from a breakfast gathering of astute businessmen here three years ago. At the invitation of Kenneth Sparks, the able chief of staff at the Federal City Council, an economic brain trust of local executives, businessmen heard a Baltimore marketing and consulting firm suggest that the entire Washington and Baltimore region be combined into a single area for economic-development promotion.

After the predictable jokes about whether it should be called Balwash, Washbal or what-have-you, not to mention some provincial doubts about Washington's need to share Baltimore's problems, participants got to thinking that maybe there might be something to it after all. And it looks as if there might. Riggs National Bank Chairman Vincent Burke Jr., who has been elected initial chairman of a group of business representatives from the two cities, is among those leaders who see many benefits in the Baltimore-Washington promotion - not just in business sales, but in jobs, leisure spending and orderly growth. So is James Rouse, celebrated developer whose new town of Columbia is right in the middle of this "common market."

This is not a common market in the European sense, but a pooling of economic promotion and planning efforts. With a starting budget of about $600,000 from corporations in the two cities and from the state of Maryland, the promotion corporation will sell its common marketing opportunities to businesses and institutions across the country as well as overseas. Once a commercial enterprise expresses some interest setting up shop somewhere in the Baltimore-Washington marketing area, then local governments can compete to land the new institutions in their jurisdictions. And there's much to be promoted in this market: The area is the country's fourth richest region in total spendable income (after the New York, Los Angeles and Chicago metropolitan areas), and it has the post of Baltimore, the federal government, a growing base of industry and financial institutions. It also has eight of the 50 richest U.S. counties in terms of average family incomes, more than 60 colleges and universities and many other amenities.

The idea may spread to include financial support from Virginia, too. For that matter, Virginia has been ahead of Maryland and the District in this kind of trade promotion worldwide, though much of it has naturally focused on the Norfolk-Richmond areas. The obvious danger is that local governments will fail to recognize the common benefits of new industry regardless of whether it locates in their particular jurisdictions. With good transportation in the region, people and jobs do - and will - cross boundaries. Among the new political leaders in the market, Maryland Gov.-elect Harry Hughes spoke during his campaign about the importance of this sort of venture. We hope that the other local figures who are about to take office in this region will join in supporting and expanding the project.