One of the most important senses to a thief is his sense of hearing. Common helmets muffled the ability to hear lock mechanisms or approaching guards, among other things. A thief wanting to protect their head and still be able to hear had the option of wearing a thieving helmet.
Thieving helmets were invented by a gnome from Waterdeep. They had special ear canals added to the structure of the helmet that amplified incoming sound to a thief's ear. These helmets sacrificed some structural protection in order to make way for the ear canals, but were better than nothing.
- Aurora's Whole Realms Catalogue, p. 17.