Josh Perelman loves a piano - not just any piano, but the one on which Irving Berlin wrote ''I Love a Piano'' in 1915. He loves it so much that he is borrowing it for 14 months.

Dr. Perelman is a deputy director of the National Museum of American Jewish History in Philadelphia, which is preparing for the November opening of its new $150 million home near Independence Mall. The piano - no high-tone baby grand, but an upright, and a distinctive one, at that - will be part of an exhibition called ''Only in America,'' about the Jewish experience in the United States.

All that explains why Dr. Perelman was standing in an alcove at Ascap's headquarters, opposite Lincoln Center, where the Berlin piano has resided for 15 years while on loan from his family. Before the movers wheeled it to a service elevator, Dr. Perelman played a few chords.

''This is one of those artifacts that was in our dreams when we started our planning: 'Wouldn't it be amazing if we were able to bring Irving Berlin's piano to the public,' '' he said. ''You can talk about him, you can show a movie about him, but to see the piano, to see the mechanism, to feel, as I did, that you're really in Irving Berlin's shoes - that's something.''

Berlin bought the piano for $100, big money for a former singing waiter in a Chinatown restaurant, in 1909. He had other pianos later on, but that one was the one he had when he wrote ''Alexander's Ragtime Band'' in 1911.

''It was part of him,'' said his daughter, Mary Ellin Barrett.

As pianos go, it is distinctive because it came with a lever Berlin could pull or push to transpose the music from one key to another - the lever moved the keys so the hammers could strike different strings. Berlin worked only in one key, F-sharp. Singers may known ''How Deep Is the Ocean'' in E-flat or ''Blue Skies'' in the sunny key of G, but they were all F-sharp tunes to him.

That gave rise to the tale Mrs. Barrett took issue with. ''The one mistake people make is they say, 'Irving Berlin only played the five black notes.' I say, no, he played in the key of F-sharp, the black-note key.'' Of course, the key of F-sharp has two white notes, E-sharp (which looks like F) and B.

But his pianos with the transposing mechanism, a mechanical contrivance that made so many famous tunes possible, ''became a legend,'' Mrs. Barrett said. She recalled a conversation she had had with the actress Alice Faye, who starred in the 1938 film version of ''Alexander's Ragtime Band.''

''She didn't have many memories except 'I remember him and that funny piano,' '' Mrs. Barrett said.

Music historians say Berlin had a name for his transposing piano: ''The Buick.'' Whether it first applied to the particular piano at Ascap or one of the others, Mrs. Barrett did not know. Nor did she know which of Berlin's pianos might have inspired the song that became ''Me and My Melinda.'' The original version was about another three-syllable mesmerizer:

Me and my piano, my piano and meWe are bound up together with sympathyMe and my pianoWe are seldom apart


PHOTOS: A piano owned by Irving Berlin is prepared for its move from Ascap in Manhattan to the national Jewish museum in Philadelphia.; The piano is distinctive because it has a lever that can be pulled or pushed to transpose the music from one key to another.; Berlin bought the piano in 1909 and wrote ''I Love a Piano'' on it in 1915. It will be at the Philadelphia museum for 14 months. (PHOTOGRAPHS BY HIROKO MASUIKE FOR THE NEW YORK TIMES)