Last night's program by the Alvin Ailey American Dance Theatre at the Kennedy Center pointed up one of the main reasons this troupe is so treasurable -- Ailey's insistence upon the most inclusive possible span for his repertory, from commonplace to the esoteric. Any company that can encompass both George Faison's "Tilt," which was a Washington premiere, and Kathryn Posin's "Later That Day," which was a world premiere, has got to be something special.

"Tilt" is a piece of pop dance trivia from a facile Broadway hand, an easy win with that large segment of the public which can make do very nicely with a display of high-energy, showbiz pizazz and an eye-catching gimmick or two.

"Later That Day" is clearly offbeat, an exploratory sort of work by a youngish, relatively unknown choreographer, and far from instantly accessible. But it's the "Tilt" kind of material in the Ailey spectrum, with its assured popularity, that makes possible the exposure of work like Posin's to Ailey's sizable audiences. Ailey doesn't have to do pieces by Posin -- he could doubtless get by with just the "Tilts." A few critics might grumble, but who'd notice? He goes ahead anyway. Ailey doesn't look down his nose at the "Tilt" stuff, nor does he have any artificial reverence for experiment perse. He just believes his public ought to be able to sample all sides of the dance experience, and that's why his company is such an invaluable national asset.

"Later That Day" doesn't strike one as a masterpiece at first viewing, but the possibility can't be ruled out -- it's strange and challenging in ways that often turn out to be signs of esthetic breakthrough.

Posin put herself ahead of the game at the start by electing to use music from Philip Glass' extraordinary score for "Einstein on the Beach," his monumental 1976 collaboration with Robert Wilson. To the mesmerizing, incantatory Glass excerpts (primarily for electric organ, chorus and solo violin), Posin has fashioned a dance work that is at the very least eventful, provocative and absorbing from a compositional standpoint. The problem is the opacity of dramatic content -- we're given hints, but not enough even to narrow the field down to interesting possibilities. Who is the Man who sits at a table consulting a huge book (a script?), sometimes directing the actions of others and sometimes joining in them? A choreographer, perhaps? Who are the Boy and Girl who sit apart on a bench and end up like lovers? And what is the Chorus, with its ritualistic permutations, and how do these elements relate to each other? In short, what, if anything, does it all mean?

As for "Tilt," it puts three women, dressed as penny arcade dolls within a pinball machine set, into incessant, trite gyration to a series of disco and soul numbers -- it's choreography squeezed from a tube.