The sign found in the ancient restorative spa town of Hamman al-Alil just south of Mosul dictates precise details of men's underwear.
Not satisfied with banning tight jeans, T-shirts, shaving and forcing families under their control to wear baggy clothing more typical centuries ago, ISIS has printed pictures of what underwear is acceptable and what is not.
The poster or sign shows a variety of shorts typical of thousands found in high streets throughout the world, from boxers to briefs.
It is rudimentary and crude, but also clear that Western style underwear must not be worn -- a red X covers each offending image.
Conservative followers of Islam typically turn to the Quran and Hadiths for guidance on what should and should not be worn. Modesty is the key, determining a general acceptance that the body between knee to above the belly button must be covered. Body contours should be hidden.
The ISIS sign depicts a man in long, baggy underwear hanging from well above the waist to below the knee. As an alternative to shorts a wrap is also shown.
Families fleeing ISIS have told CNN of the extreme punishments handed out by the terror group for anyone daring to disobey their commands -- anything from decapitation to lashes, to prison, to digging ISIS's fighting tunnels.
Although no one CNN talked to said they knew of someone who had been killed for disregarding underwear regulations, lashes or being locked up were not uncommon.
ISIS's barbaric rule extends way beyond dictating clothing -- TV and music are banned, cell phones forbidden, smoking too and no one is allowed to get their hair cut.
But this latest discovery reveals the invasion into the most intimate corner of people's lives.