Chicago PD is part of the successful Chicago franchise created by television legend Dick Wolf, who was also the brains behind Law & Order and its various spin-offs. Launched in 2014, Chicago PD was the second of the Chicago franchise series, coming two years after Chicago Fire made its debut on NBC.

Related: Chicago Justice: 10 Hidden Details About The Main Characters Everyone Missed

The show is set in a fictional 21st district which is home to the Intelligence Unit, led by maverick detective Hank Voight and patrol officers under the watchful eye of tough desk sergeant Trudy Platt.

Dick Wolf’s TV shows pride themselves on their attention to detail, and Chicago PD is no exception, especially when it comes to the costumes.

Costume Designer Susan Kaufmann Has Worked On Other Chicago Series

The costume designer on Chicago PD is Susan Kaufmann, who started her career working on TV movies before she was hired to work on the hit TV series Prison Break.

Kaufmann, who hails from Skokie in the Chicago suburbs, has now worked on all of the shows in the Chicago franchise, having started out on Chicago Fire in 2012. A lot of the characters in the Chicago franchise spend a lot of their time in uniform, but the Intelligence Unit in Chicago PD are all plainclothes cops, which gives a costume designer more room to use their imagination.

Chicago PD Is The Only TV Drama To Be Allowed To Use Real Uniforms

One aspect of the costumes in Chicago PD that Kaufmann doesn’t have to worry about is the uniforms worn by the actors playing patrol officers. The makers of the show were given special permission by the authorities at the Pentagon to use real Chicago police uniforms as costumes in the show, and even to use real badges and insignias on jackets and hats.

The Pentagon had only given this permission once before, for the 1980 comedy film The Blues Brothers, starring John Belushi and Dan Aykroyd.

Real Cops Even Star As Extras Wearing Their Own Chicago PD Uniforms

The makers of the shows in the Chicago franchise are always focused on attention to detail, and no doubt Dick Wolf and the team were delighted to get permission to use real police uniforms when dressing their stars.

Related: Chicago P.D.: 10 Facts You Didn’t Know About Hank Voight

Real police uniforms also appearances in the background, as the makers of the series often recruit real patrol officers to work as extras on the show, who then just wear their ordinary work clothes while on set. As well as adding authenticity to scenes involving a lot of cops, this also helps with continuity.

Al Olinsky's Hats Were Used To Hide His Balding Head

Alvin Olinsky, played by Elias Koteas, is an integral member of the Intelligence Unit and a close friend of Hank Voight.

He has known Voight for years and knows all about his checkered past, and even takes the rap when Voight is suspected of murdering the man who killed his son. Olinsky is rarely seen without a hat, and while this fashion choice could be a result of the cold Chicago weather, it is suggested more than once that he is trying to hide his thinning hair.

Kim's Promotion Was To Hide The Actress' Pregnancy Under Looser Clothes

In the most recent season of Chicago PD, Kim Burgess finds out that she is pregnant after a one-night-stand with ex-boyfriend and fellow squad member Adam Ruzek. Burgess ends up losing the baby in a tragic episode, but it was happier news for actress Marina Squerciati, who was pregnant for real while filming season 4 of the show.

Kim’s promotion to Intelligence, which allowed her to work behind a desk and to wear looser fitting clothes, was used to hide Marina’s pregnancy from the cameras.

Sophia Bush Loved Erin's Tomboyish Wardrobe

Actress Sophia Bush, who played popular detective Erin Lindsay until the character left Chicago PD to join the FBI in season 4, loved the dressed-down look of her character’s costumes.

Related: Chicago P.D.: 10 Things You Didn't Know About The Cast

Before joining the Chicago franchise, Bush had been one of the stars of teen drama One Tree Hill, in which her character Brooke Davis had to wear much more glamorous outfits, which involved a lot of costume fittings. When the actress joined Chicago PD, she said the number of fittings had gone down from three in a week to once every three weeks!

Replicas Of Hank Voight's Iconic Leather Jacket Are Available For Sale

Undoubtedly the most interesting character on Chicago PD is detective Hank Voight, a man with a shady past who isn’t afraid to bend the rules to catch the bad guys.

Played by Jason Beghe, Voight first appeared as part of a storyline on Chicago Fire, and he kept his iconic look when the character became the focus of Chicago PD. He always drives a matte black Cadillac Escalade, and he is rarely seen without his trademark brown leather jacket. Fans of Voight’s style can even buy replica jackets for themselves.

Olinsky Was Rumored To Wear Diapers On Stakeouts After Losing A Suspect

Olinsky explains why he wears diapers in Chicago PD

This particular costume detail was never actually seen in the show – much to the relief of fans everywhere – but it was something of a running joke among the characters on the show that Al Olinsky would always wear diapers when he was on a stakeout.

The story goes that he once lost a suspect because he was away from his post to answer a call of nature, so since then he hasn’t left a stakeout position for anything, and that adult diapers were an essential part of his stakeout kit.

The Cast All Have To Be Wrapped Up Warm For Filming On Location In Chicago

There are usually around 20 episodes in each season of Chicago PD, with all of the outdoor filming taking place in real locations around the city. It can take a long time to get so many episodes filmed, especially when production needs to coordinate so many car chases and fight scenes.

This means that much of the filming has to take place during the winter months when the temperatures in Chicago can be as low as -20°F. Little wonder that the cast is often wrapped up in heavy winter coats, gloves, and woolly hats.

Real Intelligence Teams Are Unlikely To Run Around In Bullet-Proof Vests

The action sequences in Chicago PD are part of what makes the show so entertaining. It wouldn’t be half as much fun if the Intelligence Unit spent all their time sitting in the office.

However in reality, that is exactly what intelligence officers do with their days; they analyze evidence to find suspects and then send trained armed officers to arrest them. Real intelligence officers would rarely draw their guns if they were armed at all, and certainly wouldn’t run around Chicago in bulletproof vests.

Next: Chicago PD: 5 Characters That Need More Screen Time (& 5 That Deserve Less)