When Edgar Degas was presented to Mary Cassatt, he met his match. Their artistic dialogue, close and collaborative, was that of equals. Their friendship endured until Degas’s death in 1917. The exhibition Degas/Cassatt, which opens Sunday at the National Gallery of Art in Washington, DC, presents a lively picture of their working relationship over a period of some ten years.

This museum is uniquely qualified to present this exhibition because they own 160 works by Degas, the third largest collection of Degas’s works in the world, and 119 works by Cassatt, a collection known for its depth, breadth and quality.

The notion of artistic dialogue is a useful and revealing concept. Nearly ten years ago, the Museum of Modern Art (MOMA) presented “Pioneering Modern Painting, Cézanne and Pissarro,” which explored that lengthy working relationship and friendship. However, the artistic connection revealed in “Degas/Cassatt” is decidedly different. Cassatt was never a student of Degas; they did not paint side-by-side en plein air as did Pissarro and Cézanne.

Most intriguing is the difference in gender and nationality. Degas, a man born in Paris and trained at the École des Beaux-Arts, was known for his pointed wit and sharp tongue. Cassatt, an American woman trained at the Pennsylvania Academy of Fine Arts in Philadelphia, was independent and strong-willed with feminist leanings. There are no surviving letters to chronicle their pointed exchanges, which were often recounted by friends and family. But they found common ground in their mutual respect for each other’s work.

Conservators at the National Gallery recently confirmed the most telling evidence of their close collaboration. Art historians often say that Degas added his own brushstrokes to Cassatt’s painting, “Little Girl in a Blue Armchair,” (1878), shown in the Fourth Impressionist Exhibition in 1879. Infrared imaging, conducted at the National Gallery during recent conservation of the painting, reveals the exact change that Degas made.

Originally Cassatt had drawn the floor line horizontal to the lower edge of the painting. Degas added brushstrokes that created a corner behind the blue chairs, resulting in diagonal walls and a more interesting composition. In a letter to her dealer Ambroise Vollard, who would later buy the work, Cassatt herself acknowledged Degas’s intervention in the painting, “…he even worked on the background…” The exhibition displays a replica of the infrared image that reveals this change and other changes Cassatt subsequently made.

The most intense period of collaboration was probably during their work on “Le Jour et la Nuit” (The Day and the Night), an ill-fated journal which was never published. While the exhibition features numerous etchings made specifically for the journal, the story is missing prints by Camille Pissarro, who worked closely alongside Degas and Cassatt on the journal.

Since Pissarro did not own a printing press, Degas occasionally made prints from plates prepared by Pissarro, evidence of their close collaboration. Once upon receiving new etchings from Pissarro (who lived in Pontoise), Degas replied, “I hurried to Mademoiselle Cassatt with your parcel. She congratulates you as I do in this matter.” The prints they originally made for the journal were finally displayed in the Fifth Impressionist Exhibition in 1880.

The prints in this exhibition show works in different states by Cassatt and Degas, revealing the use of similar subject matter. One of Degas’s most familiar prints is his depiction of Cassatt at the Louvre. The exhibition includes a drawing (study for the print) and a series of prints, each of them a different combination of techniques. A series of prints by Cassatt, “The Visitor,” includes the study drawing and five different prints combining numerous techniques. When the various techniques are combined with papers with different textures and colors, the number of combinations is endless.

Degas is better-known for nudes, but this collection includes two nude prints by Cassatt that are particularly noteworthy. “Standing Nude with a Towel,” a softground etching demonstrates her skill at combining light and shadow with drypoint and aquatint. The gorgeous print, “Woman Bathing” in color drypoint and aquatint, was part of a series called “The Ten,” completed in 1890-1891, after Cassatt saw an exhibition of Japanese color wood-block prints in Paris. She re-imagined the Japanese prints by using multiple copper plates for each composition, fulfilling what she had written Berte Morisot, “I dream of it and don’t think of anything else but color on copper.”

The wide variety of media included in the exhibition is indicative of the inventiveness and proficiency of Cassatt and Degas. In addition to the prints are oil paintings, paintings made with distemper (pigment mixed with glue) and metallic paint, pastels, gouache, watercolor, egg tempera (egg yolk and water mixed with pigment), drawings, lithographs and monotypes.

Degas/Cassatt, on view from May 11 through October 5, 2014, The National Gallery of Art, West Building, Main Floor, 6th and Constitution Ave. NW, Washington, DC, (202)737-4215, www.nga.gov/content/ngaweb/exhibitions/2014/degas-cassatt.html.

Ann Saul is author of a book on Camille Pissarro, “Pissarro’s Places.” More information about her work can be found at www.pissarrosplaces.com.