Sometimes, the story behind a great painting is literally found behind the painting. Consider Mary Cassatt’s 1878 impressionist gem “Little Girl in a Blue Armchair.”

Cassatt later in life recalled how her friend and older colleague Edgar Degas offered tips as she painted the oil and even worked on the background himself. The true extent of his involvement was a mystery, however, until conservators and researchers at the National Gallery of Art used x-rays and infrared imaging to discover that Degas applied his own brushwork to literally rearrange the scene, introducing wide-angle diagonals to give the work its unique spatial dynamism.

The way the 19th century greats shared in the creative process provides the theme of “Degas/Cassatt,” an exhibit at the National Gallery through Oct. 5 that reveals the degree to which the artists adapted each other’s tastes and learned from one other. The high- profile show features some 70 works in a variety of media, including pastels with metallic paint, tempera and innovative black-and-white etchings.

The by-all-accounts platonic relationship between the artists is often portrayed as one between a mentor and protégé. Degas (1834-1917), perhaps best known for his ballet paintings from the 1880s, introduced Cassatt (1844-1926) to pastels and etchings and tutored her in draftsmanship. But the young Pennsylvanian exerted her own influence on the cranky, at times misogynistic, French master through her audacious risk-taking, according to Kimberly Jones, associate curator of French paintings.

“What emerged was an understanding of two artists that were not teacher and student … but rather a dialogue between peers, between colleagues, between equals. Degas is essentially giving a friend advice and then leaves it to her to resolve the problem,” said Jones, who organized the show.

The exhibit examines the period leading up to a landmark 1879 impressionist exhibition in Paris where Cassatt made her debut to considerable acclaim. Her penchant for using unusual media helped shape Degas’ “Portrait After a Costume Ball,” a rendering of a wealthy industrialist’s wife that mixes strokes of metallic paint with pigment, glue and pastels to create a richly textured surface. Cassatt pursued similar effects at the same time in “At the Theater,” a portrait of a young woman that incorporated metallic paint, gouache and pastel.

Energized by the positive reception at the exhibition, the pair next explored the possibilities of black-and-white imagery, collaborating on a proposed journal of original prints that was never published. Working side-by-side with the more experienced Degas, Cassatt quickly learned the technique of softground etching, which resembles pencil or chalk drawing. Her “In the Opera Box” shows innovative contrasts of light and dark by obscuring the female subject’s face entirely in shadows.

Cassatt later became the model for a series of Degas’ etchings and pastels in which he portrayed her browsing artworks in the Louvre. He would go on to amass nearly 100 of her works, including multiple versions of the same prints. Cassatt returned the favor, acting as a liaison of sorts between Degas and other impressionists and wealthy American collectors.

The National Gallery is a fitting venue for the show, boasting extensive holdings of both artists. “Little Girl in a Blue Armchair” was donated by Paul Mellon and demonstrates Cassatt’s powers of observation by showing a fidgety child with her sleeping dog in an adjacent chair. Beyond the strong colors and energetic brushwork, the work effectively evokes the innocence of childhood. But recent cleaning and technical analysis led by senior conservator Ann Hoeningswald showed just how much Degas influenced the outcome.

Cassatt originally intended to use a horizontal line to mark the edge of the floor in the scene and maintain a single back wall parallel to the picture plane, according to Jones. Degas injected bold perspective by introducing a diagonal to expand the space and create a sloping wall, she writes in an accompanying catalogue. The exhibit shows how forensic studies by the gallery staff revealed strokes of grayish brown paint not found elsewhere in the picture in the corner beyond the furniture. Degas also abraded the surface, as he did in many of his own paintings, but left it to Cassatt to make adjustments.

Imagery shows that she repositioned the armless couch in the center of the picture to align it with the new interior and also tried placing the dog on the floor before returning it to its cozy perch.

The detective work showed how the artists carried out a collaborative dialogue, Jones said, and puts to rest notions that the male artist was somehow stage-managing Cassatt’s career. “Degas is essentially is giving a friend advice and then leaves it to her to resolve the problem,” she said.

“Degas/Cassatt” can be seen through Oct. 5 at the National Gallery of Art’s West Building, Sixth Street and Constitution Avenue NW. Hours are Monday through Saturday from 10 a.m. to 5 p.m. and Sunday from 11 a.m. to 6 p.m. Admission is free.