Modern menswear as we know it first arrived mid-twentieth century. For the first time, men began dressing in a way that suited the new era of transatlantic travel, wristwatches, 3-martini lunches, Playboy magazine, and jazz clubs.

While hats were still in fashion (at least until John Kennedy’s election in 1960), men of the time stopped dressing like their fathers and grandfathers. And the Number One Sack Suit, first introduced by Brooks Brothers in 1938, began to replace the stuffy three-piece pinstriped suits and tailcoats of a prior generation.

The Number One Sack Suit became the uniform of Wall Street Bankers and Madison Avenue Ad Executives (Mad Men) alike. Characterized by a natural shoulder, rolled lapels, undarted drape, soft-front construction, a single vent (as opposed to the double vents favored by British tailors) and straight-leg trousers, it represented a general loosening of the office dress code. Its unstructured tailoring was quite different from what was being produced by the tailors on Savile Row in London and in Milan and its unfussy but still clean lines make it uniquely American. Most important, the Sack Suit was available off-the-rack, and didn’t require multiple trips to the tailor.

The name may make you think of a potato sack and while it isn’t that, it does have a boxy look as compared to the hourglass shape of the Conduit Cut Suit worn by James Bond or the rockstar-inspired skinny suits that Heidi Slimane designed for Dior, Saint Laurent, and more recently, Celine. Above all, the Sack Suit achieves a comfort level that can carry a man from a board meeting to drinks to dinner to whatever comes after while still looking unruffled.

Unless you lived through the 1940s, 1950s, or 1960s, you probably got your first glimpse of the Sack Suit watching the AMC show “Mad Men”. The show even set off a bit of a fashion craze for suits with both Brooks Brothers and Banana Republic coming out with Mad Men inspired collections. But there’s more here than meets the eye. While the 1960s style menswear seen in the show is period accurate, it has also been meticulously tailored and looks far better than what most men would have worn in real life. Take a tip from the men of Mad Men and always tailor your suits, even those you buy off the rack or online. Another interesting style point to notice here is that American men in the 1960s favored crisp white (always ironed) shirts with spread collars, not the button-downs (sometimes with paisley or floral patterns) of the British Mods.

While other manufacturers have imitated its style, the original Number One Sack Suit is still available from Brooks Brothers, practically unchanged since its introduction more than a hundred years ago.