Adults Come Back to the Piano. MUSIC: ENCORE

THEY will admit, many of them, that their mothers were right: They never should have quit taking piano. So here at the Steinway dealer, they sit before electronic keyboards, hands poised, eyes glued to their music books.

At the teacher's signal, the pupils begin to play a short tune with their right hands. Out come a few clunker notes and nervous giggles. But instructor Debbie Adams reminds her pupils of the class rules:

No.1:Be proud of your mistakes.

No.2:Forgive yourself immediately.

Whether in their 30s or their 80s, more and more adults in the United States are limbering up their fingers, says Madeleine Crouch, spokeswoman for the National Piano Foundation, an organization in touch with piano teachers nationwide. According to the NPF, many of them were childhood students of piano.

Richard Walker, a 39-year-old businessman in Adams's class, studied piano as a youngster, ``until I discovered sports and girls.'' Taking up the instrument again ``has been something I've wanted to do for a long time,'' he says.

For working people, playing the piano may be a relaxing finish to a busy day. ``People don't want to sit in front of a TV set, especially if they've been in front of a computer terminal all day,'' says Ms. Adams. She lists doctors, lawyers, and a psychologist among her many adult students.

With the influx of adults, piano teachers have begun to change their teaching styles. For example, ``piano books don't have animal pictures on them,'' says Brenda Dillon, project administrator of the NPF. Adults have goals different from children's, she says. An adult may only want to learn a specific song, or Christmas carols, or hymns for Sunday School.

Adults tend to be highly critical of their playing. ``Their oral capacities are so far ahead of their physical capacities,'' says Calvin Herst, director of education at the Community Music Center in Boston, ``and that becomes frustrating for them.''

Sometimes it's good to start with group lessons, says Mr. Herst, where adults ``can see that other people have the same problems they do and that they're not dumb, after all.''

The fact that more adults are taking piano lessons can be linked to the growing number of retirees. According to the NPF, the average American today will spend 23 percent of his or her life in retirement, compared with 7 percent in 1940. More leisure time and more discretionary spending have made piano study ``the possible dream,'' says Ms. Dillon. The foundation has just produced a video of that title, to alert piano teachers to this growing market.

Rosetta Quimby, 74, began private lessons in her 60s, but dropped them. Now she's enjoying piano again, only this time in a group setting. ``You get acquainted with some nice people,'' she says, mentioning fellow pupil Frank, who is in his 80s.

Mrs. Quimby says she has a piano at home but also enjoys the electronic pianos in class, which feel and sound remarkably like acoustic pianos.

The introduction of electronic keyboards has prompted people to learn piano, says Christine Hermanson, director of the Sarasota (Fla.) Fine Arts Academy.

``They are deciding to try music because they don't have to make such a large investment,'' Mrs. Hermanson says. Rather than buying a piano for thousands of dollars, one can purchase a basic portable keyboard for a couple of hundred.

BUT the kind of instrument one ends up playing depends on one's goals, says Tom Long, director of music education at the Baldwin Piano & Organ Company. An expensive home organ with ``lots of bells and whistles'' is usually considered ``entertainment'' and played just for fun, Mr. Long says. Portable keyboards are often impulse buys, giving one the ``instant gratification'' of being able to create sophisticated, magical sounds.

An acoustic piano, however, ``is still perceived as real music,'' Long says, a ``serious'' instrument. Fans of classical music, especially, will likely opt for a traditional piano.

Mr. Walker, on his way out of class at M. Steinert & Sons, says he enjoys jazz recordings and would like to be able to play jazz on the piano. Having played some clarinet, he thinks piano is easier - ``the keys are right in front of you,'' he explains.

Not only is the piano accessible, but ``it's basic to American culture,'' says Dillon, who cites a number of popular movies and television commercials that feature piano melodies.

In addition, students have told her that playing the piano brings ``peace and order'' into their lives, she says. ``All of us have a need to express how we feel, and the piano is one of the most tangible ways to do that.''