STEPHEN SONDHEIM is the El Cid of the musical theater. He oversteps bounds, and the audience excuses him because his work is so good. The musical quality of Side by Side compels audiences to overlook the fact that the evening constitutes a self-conceived monument to the artist. There is nothing new or creative in Side by Side, a revue of songs from musicals for which Sondheim wrote lyrics and some of the music, punctuated by explanations and anecdotes slipped in by the show's narrator.

The performers admit from the outset that they provide nothing more than sheer entertainment, and as "a musical entertainment" the show is a great success. The first song promises "tragedy tomorrow, comedy tonight," and with that reassurance, the audience relaxes, relieved that it will be subjected to neither emotional conflict nor a plot that would demand their close concentration.

If Sondheim has succeeded by overstepping the bounds of creative humility. Side by Side in and of itself works by taking the opposite approach. The show is professional but low-key, and unpompous narrator Patrick Harris tells anecdotes from the lyricist's life (memorable things Sondheim said, stories from his youth, the tale of his early humiliation by master songwriter Oscar Hammerstein) and then carries over the casual tone into his introductions of the next few songs.

Harris avoids talking down to the audience, offering the minimal background necessary to understand the context of each number while loosely tying groups of songs together under themes such as marriage. His tone is not overly intimate with the other three performers, and he speaks only long enough to give the audience a break once in a while from the 28 musical numbers that race by.

James Bundy, Maggi-Meg Reed, and Katharine Kean burst onstage in "Comedy Tonight," which immediately satisfies with its snappy lyrics and fast beat. The trio maintain professional distance from the audience, and, here again, director David Frutkoff's restraint preserves the professional atmosphere.

Nearly all of the numbers are light, testing the performer's versatility but not their depth. Reed fills the entire room with her voice and her presence, nearly eclipsing others on stage. She is best in solo, and often sings the part of the jaded or the jilted woman.

She musters all of her commanding appeal as a brothel madam singing "I Never Do Anything Twice." Perched on a stool by the piano, a black lace shawl draped over her shoulders. Reed stretches out her legs, throws back her head and recounts escapades with kinky abbotts and other unusual clients, always returning to the admonishment that she never repeats her experiences. Her husky voice seeps into the darkness around the spotlight, reaching the back rows with its delicious bawdiness.

REED BRINGS an air of confident, but never smug, sophistication to her numbers. However, the highly emotional style that succeeds with songs such as "Send in the Clowns," becomes familiar by the end of the evening, and is obviously unsuited to softer numbers. Kean's quieter voice and more focused performing style provides a contrast to Reed's occasionally overpowering delivery.

Kean fills the stage without overrunning it. A little too soft in "Anyone Can Whistle"--from a Sondheim show that closed after only one week--she treats this thoughtful, almost philosophical song with the same muted projection and shyness that are more appropriate to the lighter "Broadway Baby," a prayer for theatrical hopefuls, waiting for that one big chance.

Kean highlights the humor of "Barcelona," bringing to it a similar interpretation. In this familiar song, Bundy begs his stewardess bedmate not to leave the morning after, all the while hoping she will. She cooperates by insisting that she must fly on to Barcelona. At the last minute, she agrees to stay, much to his chagrin. In this piece, as throughout, the two use only one or two props, and remain clad in streetclothes. They pick a theme begun some time earlier in a musical that the audience, of course, doesn't see. But even without a developed plot they convey with perfect clarity the confrontation of two strangers who wake up together uncertain of each other's motives. Kean and Bundy work best together, always creating a dramatic situation rather than simply singing a duet.

The two women of Side by Side revolve around Bundy as he balances the trio, providing a foil for their criticism of men. His rich, strong voice and bearded mugging make songs like "Could I Leave You?" (In a minute!) hilarious. He moves a little heavily about the stage in the beginning but catches up to the very fast, controlled pace of the other two in the second half of the show.

Bundy provides some of the evening's most comical moments and is especially funny in "You Gotta Get A Gimick." He leaps out on stage in a tuxedo and a red tasseled loincloth embroidered with gold. Lunging left and right, he quotes Shakespeare as he bumps and grinds to emphasize each couplet.

The evening ends surprisingly quickly considering the show's line up of 28 songs. The audience hardly has enough time to consider what a success has resulted from Sondheim's self-congratulatory musing. In the end, it didn't matter how El Cid achieved his greatness, and it doesn't seem to matter how Sondheim arrives at his. It seems a bit of a shame, though, that while some audiences enjoyed his talent when he first presented these songs, his earlier musicals have not achieved the popularity of Sondheim in repetition.