From the opening frames of HBO’s series “The Gilded Age,” the music of composers Harry Gregson-Williams and Rupert Gregson-Williams gives viewers a sense of the ambition and drive of “new money” in conflict with the “old money” of 1880s New York.

The brothers — Harry based in Los Angeles, Rupert just outside London — generally work on different projects, and most often for features (Harry’s credits include “The Martian” and “The Last Duel,” while Rupert’s include “Wonder Woman” and “Aquaman”).

But both are past Emmy nominees (“Electric Dreams” for Harry, “The Crown” for Rupert) and this is their second TV collaboration, after 2019’s Hulu series “Catch-22.” Given the sheer volume of music demanded by “The Gilded Age,” nearly six hours for the 10 episodes, they were happy to reunite professionally.

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“It was good to have two sets of hands on it,” says Harry. “Upwards or 35, sometimes 40 minutes of music [per episode] had to be written, approved, orchestrated and actually played live. It was a big job, and it stretched us.”

Finding the appropriate musical style was an intriguing challenge, says Rupert: “Coming out of the Americana period and into a certain period of New York, we used an orchestra, which was conventional, but then we brought in unconventional sounds that occasionally surprised, and helped to tell the story of old and new money meeting and clashing.”

Those surprising sonorities included hammered dulcimer, ukelele and a Finnish stringed instrument called the kantele. Producer-director Michael Engler “really enjoyed those colors and encouraged us to use them,” Rupert adds.

The main title theme, released by WaterTower Music, is a dynamic piece set against images of the railroad, stocks and bonds, top-hatted men, grand staircases and sparkling chandeliers, all designed to evoke the elegance of the era.

“We felt it needed sweep and elegance and size and scope,” notes Rupert. It is the theme for the Russells, the railroad tycoon and his social-climbing wife (Morgan Spector, Carrie Coon). “We thought that the main theme should be about the new money.”

A second major theme represents the van Rhijn family (headed by Christine Baranski), the old-money Manhattan dwellers who resist change and resent the newcomers. “We spent a considerable amount of time, a few weeks, to experiment and throw thematic material around,” says Harry, “to clarify and help tell the story. The characters have layers, and take off in multiple directions, so there’s always hustle-bustle on the screen, and we reflect that, musically.”

Although created and co-written by “Downton Abbey” author Julian Fellowes, the filmmakers weren’t looking for similar music, the brothers said. The orchestra is larger (48 players, compared to 35 for “Downton”) and the generous HBO budget allowed them nine hours of recording for each episode.

The score was truly co-written, the brothers say. One would write thematic material and send it to the other. “We messed with each other’s ideas,” Harry reports. “We shared files with each other, talked about what we liked and what we didn’t think would be useful.”

Harry conducted the Los Angeles ensemble while Rupert monitored the recordings, in real time, from his studio in England. The entire process took about four months, they said.

As for their unusual billing (“Music by the Gregson-Williams Brothers”), it was an improvement over an earlier version, which “looked like ‘Mr. and Mrs. Gregson-Williams,’ which felt wrong,” Rupert cracks. Harry suggested “the Gregson-Williams Brothers,” which they preferred as “kind of funny and nice.”