In July 1940, the British government established a secret intelligence agency called the Special Operations Executive (SOE). After the fall of France and the evacuation of the British forces at Dunkirk, Britain needed a way to undermine the Axis powers from within. The SOE was created to train and manage covert agents who would infiltrate enemy territory for purposes of reconnaissance, subversion, and sabotage. Thousands of spies would eventually penetrate every theater of World War II, from Poland to Ethiopia to Burma, with most serving in continental Europe. The organization’s greatest presence was in France, where the SOE dispatched 1,800 agents from May 1941 to September 1944.

The SOE’s spies all posed as inhabitants of the places they infiltrated—up to two-thirds of the agents actually were natives of the countries in which they worked. Being able to pass for a local depended on more than a flawless ability to speak the native language. It also meant looking the part.

National and regional differences in fashion were more pronounced in the 1940s than they are today, as markets were less globalized and many people still made their own clothes or acquired them from local tailors and seamstresses. Spies needed to blend in, and to do that, they needed to wear what the locals were wearing. At first, the SOE sourced outfits, shoes, and suitcases from secondhand shops or bought them from refugees who’d fled continental Europe for the British Isles. But these sources were soon depleted, meaning the SOE had to begin manufacturing their own “authentic” European clothing and luggage to outfit their agents. The SOE outsourced much of this production work to clothing companies owned by refugees, who were already well-versed in the sewing styles of their native lands.

Claudia Pulver, a seamstress from Vienna, was working for a small garment-production company in central London called Loroco, Ltd., when she suddenly found herself a costume designer for Britain’s spies. “To start with,” Pulver told an interviewer for the BBC2 documentary program Secret Agent Selection, “we got old shirts from refugees and we took them apart, looked at the various collar shapes that were fashionable at the time, looked at the way they were manufactured, looked at the seams.”

Something as simple as the stitching on a seam advertised to the keen eye whether a garment was British or French or Dutch. “There certainly was an enormous difference between the side seams that were made on the Continent and those that were made in England,” Pulver recalled. “We did what was called a French seam.” (A French seam is sewn in two steps, so that the raw edges of the fabric are totally encased by the stitching.) Pulver and her colleagues took note of regional differences among the refugees’ clothing and created cardboard patterns to recreate the styles later.

When possible, SOE tailors used fabric smuggled out of Europe or repurposed from existing clothing taken from refugees or secondhand shops. Labels were also transplanted from such items or copied in meticulous detail. For tailored suits, an imitation tailor’s slip was placed inside the left jacket pocket. Thread had to be the right thickness, and preferably sourced from the region where the agent was going. Plain bone buttons worked best for most clothing items, although suspender buttons were sometimes stamped “elegant,” “mode de Paris,” or “for gentlemen,” as was common on both German and French trousers. British-made zippers bore the brand name Lightning, which had to be carefully ground off the metal pulls with a dentist’s drill.

“Meticulous care was taken that every article of clothing and all accessories should be an exact replica of items manufactured in France,” recalled Claire Wrench, an SOE worker at Orchard Court, a mansion in London where agents got ready before parachuting into France. “Even the buttons on the men’s suits needed to be sewn on in a special French style.” In France, buttons were usually threaded in two parallel lines, rather than a crisscross pattern. Such differences, which in other circumstances might seem tiny and insignificant, could give a spy away. For instance, in the 1940s, many men still wore detachable collars, which fastened onto the shirt with small studs, like cufflinks. In Britain, the stud hole on the back of the collar was a vertical slit; on the continent, it was horizontal—something a German officer could easily check.

“It was a very, very thorough job, because a man’s life depended on it,” observed Albert Adlington, an SOE technician. Adlington’s job was to “age” the new clothing, accessories, and luggage being made for covert agents’ use. “There were no new suits in Germany, only for the real higher-ups, so the suit had to be aged,” he explained. To make a suit look used, Adlington and his colleagues had a simple solution: wear it, night and day, for a week. “It stunk to high heaven at the end of it ’cause you never had a shower or nothing!” he remarked. Once the suit had begun to crease from wear, a technician would rub a thin layer of Vaseline over the creases, to darken and set them. Then, to create the appearance of wear and tear, he’d dust the fabric with rottenstone (an abrasive powder used in woodworking and metalsmithing) and take fine sandpaper to the lapels. Workers would even intentionally make holes in the material, just so they could darn them, SOE seamstress May Shrubb told an interviewer from the Imperial War Museum. Everything had to be “dirtied down,” she said, because both clothing and shoes were carefully rationed during the war, making anything new look suspicious.

At the beginning of the war, the SOE’s seamstresses and tailors were making clothing to order. “We had all these lovely young men coming in,” said Claudia Pulver, “all these handsome dashing officers from all sorts of countries like France and Canada. We had to take their measurements and we made about four or five shirts for each of them.” But after a while, the SOE began amassing a collection of clothes, styled for various regions and aimed to bolster different cover stories.

At the height of operations, the SOE outfitted 16 agents a day. By 1944, the organization was dispensing more than 90,000 articles of clothing annually. But the SOE only kept a stock of clothing on hand for their male agents, as fashions for women varied too much, not just from country to country but from town to town. There were also far fewer female agents—approximately 50 women out of 1,800 total agents in France—so female spies got bespoke outfits.

Agents also changed their hairstyles based on local trends, and the women adapted their makeup. Yvonne Baseden, a wireless operator in France, told historian Juliette Pattinson that her “hair was cut specially” before setting off for her mission. And Yvonne Cormeau, another covert radio operator, recalled being told not to dye her hair, have a manicure, or wear noticeable makeup, as such glamorous touches would be out of place in the rural area where she was being sent. Men altered their facial hair—either to fit in with the locals or to change their appearances. Gaston Cohen managed to escape France when agents in his Paris-based covert network, Juggler, a subdivision of the Prosper circuit, began to be arrested. He then returned for a new mission with a freshly grown moustache. In contrast, Kenneth MacKenzie, an SOE agent in rural France, later recalled having to shave off his own moustache. Speaking to an interviewer from the International War Museum, MacKenzie observed that, if he’d kept his facial hair in the region where he worked, “I would be really advertising a British officer, so I had to cut my moustache off. Very sad!”

And the SOE didn’t just demand that their agents make external changes to their grooming habits: London dentists even removed spies’ British fillings and replaced them with gold, as was used by French dentists.

But even with such careful attention to detail, the SOE couldn’t always appropriately furnish Britain’s covert agents. One problem was supply: French shoes, for instance, were hard to come by, and more complicated to make properly than a shirt or a dress. In her memoirs, Pearl Witherington, the leader of the WRESTLER circuit in central France, recalls scrambling to find a pair of shoes that weren’t too conspicuously English before leaving the UK, eventually obtaining “a less obvious pair, little material boots with cork soles.”

Brian Stonehouse, in contrast, didn’t realize how noticeable his English-made shoes were until he was already in occupied territory, and an old French woman struck up a conversation about his “beautiful English shoes” on a train packed with German soldiers. Outfitted from head to ankle in clothing made by SOE tailors in the French style, Stonehouse and his handlers had somehow forgotten his feet. Luckily, the German soldiers took no notice, and Stonehouse played it cool with the old woman, then replaced his shoes as soon as humanly possible.

Another problem was repetition: The SOE tailors were manufacturing thousands of clothing items each month, but the various items couldn’t look too similar. Francis Cammaerts, leader of a clandestine circuit in southeast France codenamed JOCKEY, objected in a mission report that “Clothes were given on a definite pattern and often repeated,” which could make agents conspicuous. He complained, in particular, that agents sent to work with the French Resistance wore “exactly the same kind of shoes, which were of a special colour and marking,” as agents assigned to other covert circuits. If these had been identified by the enemy as “spy shoes,” it would have been disastrous for the British.

In one instance, an SOE saboteur and his radio operator were accidentally dressed in identical outfits before being parachuted into the Netherlands. Imagine one tall, lanky man and another short, plump man jumping out of a plane with matching socks, shoes, shirts, ties, raincoats, and briefcases. The spies managed to change before their matching get-ups attracted attention, but both men were captured five months later, when the German detected their radio transmissions.

M.R.D. Foot—the official historian of the SOE, who was allowed access to government archives 33 years before the documents became public—writes of another close call in his book Resistance. Foot reports that a refugee tailor employed by the SOE had brought two bolts of striped cloth to England when he fled continental Europe, which the SOE used to equip each male agent destined for the Continent with a pair of striped pajamas. One spy put on his PJs his first night in occupied territory, and a local contact recognized them immediately as the SOE’s “house pajamas,” advising the agent to replace them right away: There were a suspicious number of men in the area with the same ones.

Historians don’t know if any SOE agents were ever betrayed by their clothing, snatched up by German soldiers or local Nazi sympathizers due to a mis-sewn button or an overtly English pair of shoes. Even some agents who were captured and sent to concentration camps were not identified as British by the Nazis, successfully pretending to be black marketeers, locally recruited resistors, or unwitting Frenchwomen who were caught up in something they didn’t understand. Though passing as natives did not protect these agents from torture, they were less likely to be executed than if they had been identified as British spies. Thousands of SOE agents, clothed in garments designed to disguise them, successfully infiltrated enemy territory and returned home alive—thanks, in part, to the work of seamstresses and tailors who imitated foreign clothing right down to the button holes.