Mr. Heffernan, 40, approached the exercise in a way that befits an M.B.A. who had spent a year working at Bain Consulting. “We looked at the numbers, which were just staggering,” he said. “Socks, particularly colored socks, were up, gloves were up, scarves were up, even ties were up.” Everything was up, that is, except belts. “We thought, surely this is a sleeping giant,” he said.

In January, the couple launched Beltology, an online-only brand devoted to giving the least-noticed, least-talked about and least-fetishized accessory in menswear its proper place of worship. “We want to do for belts what Swatch did for the wristwatch back in [1983],” said Mr. Heffernan.

Mr. Heffernan and Ms. Lundberg, 26, have developed what they believe to be the Platonic ideal of the men’s belt, based on a popular Italian style. Woven from strips of elasticized rayon, polyester or waxed cotton, leather-trimmed and with hardware made of the metal alloy Zamak, the 1- to 1¼-inch-wide belts are available in 50 iterations—some solid, some multicolored and patterned. They sell for between $45 and $65, about half the price of pieces by cult woven-belt purveyor Anderson’s of Parma, Italy. A similar version in blue cotton by Paul Smith retails for $125 on the men’s fashion website Mr Porter.

“‘We looked at the numbers. Socks were up, gloves were up, scarves were up, even ties were up.’ Everything was up, except belts.”

“We believe that ours is by far the best belt for two reasons,” Mr. Heffernan said. “Number one, there are no belt holes, so from a sizing point of view we’ve basically covered the whole market with just four sizes. Number two, it stretches by up to 25%, so whether you’re standing or sitting, whether you’ve put on a bit of weight or lost some, it adjusts to your body.”

Mr. Heffernan and Ms. Lundberg’s product is inarguably solid, but whether belts are the next frontier in men’s accessories remains to be seen.

Brian Trunzo, co-owner of downtown New York menswear mecca Carson Street Clothiers, thinks modern tailoring might be to blame for flat belt sales. He said that many of his customers have been dispensing with belts entirely, particularly when wearing tailored pants with adjustable side tabs and back buckles. “Belts have been—to use a phrase I hate—not really on-trend,” he said.

When he isn’t being interviewed for newspaper articles about them, Mr. Trunzo said he rarely thinks about belts and believes that few men do. “Every guy has his good trusty belt he’s had for years,” Mr. Trunzo said. However, he added, his store does a decent trade in belts, particularly one of his team’s own design.

look like heavyweight championship boxing belts because they’re so thick and rough,” he said. “Our belt is only 1-inch [wide] and made of alligator, with a brass buckle. It’s sleeker and a little more elegant.” (It’s also $425, though there’s a version in bridle leather for $125.)

Andy Spade, the force behind the boutique branding agency Partners & Spade, is at a loss when asked to explain why belts have remained on the sidelines while extras like pocket squares and striped socks have become style-blog fodder. “I guess no one’s really put them in front of men yet,” said Mr. Spade, adding of Beltology: “It’s a super smart idea, though. Ralph Lauren started with ties, right?”

Ever the early adopter, Mr. Spade, the founder of menswear label Jack Spade, is himself a belt aficionado, and estimates that his collection numbers around 200. “I’m a belt lover. You might even call me a fanatic,” he said. Mr. Spade is particularly partial to Indian beaded belts and those with silver plaque buckles that say things like “Loser” or “666.” “Like ties,” he said, “belts are a way for men to express their personality without being too crazy.”

“We don’t have as many accessories to work with [as women do], so a belt can be an opportunity to really punctuate an outfit, and to help create a signature style,” observed Bruce Pask, men’s fashion director at Bergdorf Goodman. “I wouldn’t count the belt out,” he added. “[It’s] held up many a gent’s trousers over the course of hundreds of years and outmaneuvered braces and suspenders.” Mr. Pask pointed to stronger sales of woven leather belts by Giorgio Armani, Bottega Veneta and Bergdorf Goodman’s house label.In the ever-expanding menswear universe, the notion that belts could become the accessory du jour makes sense to other insiders, too.

GQ fashion director Jim Moore said that the men’s fashion magazine is “back in full-throttle belt mode,” after several seasons of jettisoning them from his pages. In last year’s April issue, he ran a story featuring over 30 styles, including a striped ribbon belt, a tooled leather version, one in woven leather and another with colorful Indian beading. “The belt is your new pocket square,” Mr. Moore said. “It’s your new accent piece that can really make or break a look.”

A few options to pick from: https://www.tomjames.com/accessories/belts/