Attitudes.

Thirdstream dance is more easily defined than traced. I find it easier to define than "postmodern" dance, which always seemed a vague nomenclature to me, or even that favorite, passed over from art criticism, "postexpressionist," especially as so many people who use the term are uncertain about precisely what defines expressionism. Compared with such lexical vagaries, thirdstream is almost self-defining: It is dance that finds a certain common ground with both modern dance and classical ballet, performed usually in ballet slippers but occasionally barefoot or even in toe shoes.

But where did it come from? Who started it? It's difficult to pin down. Yet I think we might make a useful beginning with the Nederlands Dans Theater in the early 1960s, and zero in on the late Glen Tetley, the American choreographer who, like fellow-American John Neumeier, is markedly better known in Europe than the United States.

Thirdstream dance is not just a crossover mix of the two predominant theater dance genres of the past century. Classical companies started sizing up modern dance as early as 1940, when Richard Pleasant of Ballet Theatre sent out feelers to Martha Graham and Doris Humphrey. Valerie Bettis worked with the Ballet Russe de Monte Carlo in 1947, the same year that Merce Cunningham created The Seasons for Ballet Society. In 1970 Jose Limon gave The Moor's Pavane and The Traitor to ABT, and the floodgates opened, with Graham, Cunningham, Paul Taylor, Alvin Ailey, Murray Louis, Kurt Jooss, Twyla Thaw, and many other moderns working with classical companies, which were desperately eager for creativity and finding precious few suitable recruits within their own ranks.

But not too many of these works, Tharp's apart, were genuine thirdstream ballets. They were basically modern dance works adjusted for the differently muscled feet and cleaner soles of classical dancers. Perhaps hints of the first thirdstream pieces came even earlier, quite unexpectedly from England, with Tudor's 1937 Dark Elegies and his neo-expressionist 1938 Judgment of Paris. Tudor was aware of the German moderns, so these ballets did not exactly run in from left field. Then, in 1940, arch-classicist Ashton staged his extraordinary Dante Sonata, where the women, led by Fonteyn no less, went barefoot.

But I still stubbornly think thirdstream dance found its beginnings in The Hague, that staid capital of the Netherlands. The Nederlands Dans Theater was founded in 1959 by Card Birnie and the American teacher and choreographer Benjamin Harkarvy. In 1960 the young Dutchman Hans van Manen enlisted, later to be joined by the Americans John Butler, Glen Tetley, and, on a visiting basis, Anna Sokolow. Harkarvy was a pure classicist; Butler and Sokolow were moderns. So it was really van Manen and the more experienced Tetley who came to formulate the emerging aesthetic of thirdstream dance.

It was an exciting time. I often saw the Dutch company in England and Holland. Tetley's growing influence was reinforced when in 1966 the British choreographer Norman Morrice, himself Graham-influenced, reinvented the old Ballet Rambert into the Rambert Dance Company, with Tetley making a major contribution. Tetley was a remarkable man, an intellectual with the sensibility of an artist. His first stint with the Dutch had started in 1962, the year he created Pierrot Lunaire, his choreographic breakthrough. Although for a time he sporadically maintained his own American company, from then on he made Europe his creative base.

Many of Tetley's works found their way into both The Royal Ballet and ABT's repertoire, notably the 1973 ballet he made to Poulenc's Voluntaries, originally for the Stuttgart Ballet as a memorial to John Cranko. Its sweep and power, its use of classical technique underlayered with a modern dance spirit, made it a controversial template for thirdstream dance. It was typical of the style which first inspired that fascinating wave of European choreographers in the 1970s, including van Manen, Rudi van Dantzig, Mortice, Christopher Bruce, Jiri Kylian, all with a classical background.

The worldwide effect today of that style has been considerable, even if largely subliminal. It is an influence that, I believe, has suggested to some modern dancers a freer approach to technique, closer sometimes to classical dance than the old cardinal virtues of the Graham and Humphrey schools. To classical dancers this very same influence opened up the possibilities of a more emotive expression within the balletic vocabulary. Here then were the beginnings of a fusion, the start of a thirdstream that doesn't replace, but extends, the other two.

Senior Consulting Editor Clive Barnes also covers dance and theater for the New York Post.