BOSTON - Inside the shamrock-laden, plush-carpeted confines of the Boston Celtics' locker room Tuesday night, Paul Pierce scooted out of one closed door, offered a polite hello, then slid behind another.

A minute later, Pierce reappeared, knowing those waiting likely waited for him.

Of that polite hello, Pierce said, "I may not have meant it, but it's Christmas, and I chose the high road."

Pierce wore a confident smile, the kind you'd put on if you played for the Celtics - the reigning NBA champions, winners of, in that moment, 18 consecutive games.

Then Pierce disappeared behind the first wooden door.

Three hours later, the Celtics defeated the 76ers, 110-91, for a franchise-record 19th straight victory.

Like rock stars, the Celtics walked around TD Banknorth Garden. Like the best team in the NBA, they walked over the Sixers. And the question, like another defender, trailed each green-and-white clad Celtic: "How did this happen?"

How did the Celtics go from easy pickings to nearly impossible to beat?

And why haven't the Sixers - whose 2008 off-season moves were compared to the Celtics' 2007 moves - had the same success?

Boston finished the 2006-07 season with more losses, 58, than any other Atlantic Division team. That summer, the Celtics added power forward Kevin Garnett and shooting guard Ray Allen, as well as some supporting players such as James Posey and Eddie House. They were anointed the NBA's most likely to succeed and then followed through on that prediction with a championship.

After their makeover, the Sixers started 9-14, fired head coach Maurice Cheeks, then lost newly acquired power forward Elton Brand, the man to which they had pinned their hopes, to a dislocated right shoulder.

On Tuesday, after discussing Cheeks' firing, Boston coach Doc Rivers said, shaking his head, "We were fortunate last year to get off to a good start."

Rivers meant that he was fortunate. If the Celtics hadn't, he, like Cheeks, could have been the man offering his final thoughts on what could have been.

"You make a lot of changes, and everyone tells you how good you should be, and you don't have time to work with those changes, that's tough," Rivers said of Cheeks and the Sixers. "And the changes they made were huge."

What seems to be the difference is compatibility.

"The big thing about the pieces they added is they all complement each other," Tony DiLeo, the Sixers' new coach, said of the Celtics. "They have a shooter in Ray Allen, a scorer that can score in Paul Pierce. And the big guy, Kevin Garnett, is a facilitator. It's not like you have three scorers that are all hunting shots. They all complement each other, and that is the greatest thing."

Instead, the Sixers have a swingman - Andre Iguodala - who has played most of the season at shooting guard despite his strengths not including shooting, and a starting center - Samuel Dalembert - whose minutes are usually hodge-podged among two or three other players.

During most of the Sixers' games, the group looks not like a polished unit but rather an incomplete mosaic, always struggling to find that final piece."

And once the ball had been tossed Tuesday night, DiLeo's words about the Celtics - true before - became action. The Celtics moved the basketball like they were being paid by the pass. Allen pump-faked a Sixers defender, drove the baseline, seemed to be rising to shoot, then skipped a blind bounce pass to a waiting Garnett. How Allen knew Garnett would be exactly there requires a chemistry not yet mixed by the Sixers.

"They have a complement of players that have played the game for a while, and everybody bought into the system," Sixers guard Willie Green said.

That leaves the Sixers with three answers: They don't have complementary pieces; the pieces they have are too young; those pieces haven't bought into the system.

Did the Sixers believe their summer moves had put them in conversation with Boston?

"Well, that was our goal," DiLeo said. "Of course, it takes time on the court. But, yeah, we felt adding Elton was the piece that would put us in that direction. But we're not a finished product. But definitely, we felt Elton was that piece. We feel Elton is that piece."

Not yet.