If you're under 21 or if you are 21 but don't look it, its going to be increasingly difficult to buy liquor in the vicinity of the Square.

Liquor stores and bars have begun to demand a Massachusetts Alcoholic Beverage Control Board Identification Certificate before they will serve marginally legitimate customers. The Certificates became available May 1 and already the Varsity Liquor Store will accept no other document as definitive proof of age.

The Certificates were authorized by the General Court last year and are available locally from the Cambridge License Bureau in City Hall. Liquor stores still have the option of requesting proof of age but, as in the past, they risk a $500 fine and the possible loss of their license by serving minors.

The Certificates include a 1 1/2 in. by 1 1/2 in. photograph and thus provide a positive method of identifying the purchaser. For this reason, liquor store proprietors are generally enthusiastic about the new cards.

To obtain a certificate the applicant must go to City Hall in person with $2, a naturalization, voter registration, or birth certificate, and two photographs. The cards are issued on the same day application is made.

A quick survey of liquor establishments in the area yesterday netted the following information: Cronin's will insist on the Certificates from all questionable customers in "a day or two." The Hamilton Liquor Store and the Oxford Grille plan to require them "in a couple of weeks" and the Wursthaus and the Harvard Provision Co. on June 1.

A spokesman for the Hasty Pudding Club said the Certificates will be required "eventually." The Yard of Ale, however, plans no change in its policy of not serving questionable customers at all. "As long as the burden of proof is on the establishment in this state," the manager said, "we won't serve anybody who doesn't look like he's 21, no matter what he's got in his wallet."