In ballet “classic” and “19th century” are almost synonymous. Core essences of ballet — academic virtuosity, big-theater projection, formal grace, boy-hunts-girl romantic drama and spectacular ceremony — were forged in the era of 19th-century Romanticism in ways that audiences continue to respect. A good “Giselle” or “Swan Lake” can still take us to the heart of the genre.


This year, like every other, the big ballet spring season brings to America, and especially to New York, a series of 19th-century classics almost all attributed chiefly or partly to the choreographer Marius Petipa (1818-1910). The list is short. It varies little. Yet most of these ballets, as staged today, are laden with cliché; and much of what now bears the Petipa brand name has actually been rechoreographed by his successors.


In recent years, however, scholarship has improved our understanding of Petipa’s work. One example arrives in New York on Sunday and Monday, when Doug Fullington of Pacific Northwest Ballet and several dancers from that Seattle company present three excerpts from Petipa choreography at the Guggenheim Museum as part of its Works and Process series. The excerpts are as they were notated during Petipa’s lifetime. At the same time we can see the juggernaut of big ballet companies carry on with their quite different, and usually more hackneyed, renditions.


 To give you an idea of Petipa’s enduring influence over today’s repertory, let’s look at this year’s statistics. Half of American Ballet Theatre’s eight-week season at the Metropolitan Opera House, which begins on Monday and runs through July 7, will be devoted to these 19th-century ballets: “Giselle” (May 15 to 21), “La Bayadère” (May 22 to 28), “Swan Lake” (June 25 to 30) and “Le Corsaire” (July 2 to 7). The Paris Opera Ballet follows at the Met (July 11 to 22): 6 of its 12 performances will be of “Giselle.” Meanwhile the Bolshoi Ballet tours other North American cities (Toronto, Ottawa, Washington, Los Angeles) with three classics: “Swan Lake,” “Coppélia” and “Don Quixote.” All these ballets come to us by way of Petipa.


No 20th-century choreographer — not even George Balanchine, who often spoke of Petipa as the prime mover from whom he derived much of his work — has ever dominated the art form internationally as much as Petipa still does. Yet one of the biggest of the many puzzles that surround this man is: What did he actually choreograph? Quite often what we’re shown under the name of Petipa turns out to be some mid-20th-century revision.


What kind of revisions do I mean? Let’s go to the most famous of these ballets, “Swan Lake,” and take the end of the adagio first section of the grand pas de deux that Petipa choreographed for Odile and Siegfried (the number wrongly called “Black Swan”).


Here’s what happened in the 1895 St. Petersburg production, the staging that has become the basis for almost all others since: Odile (who for the ballet’s first several decades didn’t wear black and was never a swan) and Siegfried are dancing together for the first time, though he thinks she is Odette (who wasn’t a swan when he fell in love with her in the previous act). Siegfried used to end on bended knee; and she, after holding both his outstretched hands as she took a deep arabesque penchée, would then clutch his knee with both hands, while her leg maintained its upward line.


That’s what the notation for the 1895 original shows; and it’s what, until the mid-1940s, the Vic-Wells Ballet (today’s Royal Ballet) used to dance. You can find it in photographs of Margot Fonteyn and Robert Helpmann. It’s certainly odd: nowhere else in traditional ballet does the ballerina hold her partner’s knee while extending her leg in the opposite direction.


You can see why ballet companies today have sought an alternative to this strange image. American Ballet Theatre and most Russian companies usually conclude with a partnering pose that is actually the diametrical opposite of the 1895 one: Odile and Siegfried are both upright, both facing the same way, she with one raised leg possessively angled around his body.


The old knee-clutching pose is remarkable because it’s unique; Petipa seems to have had something specific in mind. What do we think that was? “Swan Lake” was originally set in the Middle Ages; the way the prince kneels to the enchantress Odile is an image of chivalry. This belle dame sans merci has him in thrall: hence the grip of her hands, the fixation of her regard, the gestural implication of her arabesque. That line of her leg, after all, is the very same with which Odette, the ballet’s true heroine, began her leave-taking of him at the end of the previous act.