A Cap
Caps are metallic mesh placed at the top of human's heads. Their purpose is to greatly limit the mind, making those capped docile and obedient to the Masters.
History[]
Soon after discovering Earth, the Masters started studying the human race and learned about their psychology and brain structure. Based on this knowledge, they were able to hypnotize many of them through The Trippy Show. They gave the hypnotized people temporary helmet-like Caps to secure control over them. These Caps were removable, although such an act left the human depressed and disoriented.
After the Tripod Invasion was successfully completed, the Masters started using permanent Caps, which they attached to all humans in a Capping Ceremony performed on 14-year-old children. The caps are described as a spider web-like wiring that allows hair to grow through and conceal it. After a while, almost all humans wore Caps.
These new caps were smaller, more intricate, and attached to a human's scalp. Once placed, these Caps could not be removed without killing the human. The Caps made the humans obedient to the Tripods and suppressed their creativity. A Capped person would not ask questions or doubt the status quo. Will noted that those capped (unlike the older version) were still able to form close bonds and care for others. Many were genuinely kind and caring people; providing such behaviour did not conflict with the tripods orders. This is presumably because once rule was established the Tripods were not present for much of everyday life save special occasions like capping day.
Curiously, the caps did not entirely suppress criminal actions, as there were prisons for thieves and murderers.
Will and Fritz discovered in the third book that if they could shut the Pool of Fire down, it would leave the Caps inactive and freed the humans of their mind-control. This implies that the Caps received some kind of signals from the Tripod Cities and were connected to them.
The human resistance was able to attach false caps so they could move freely among the capped without suspicion. Agents with false caps posed as vagrants to scout the countryside for new recruits, or as spies to infiltrate the cities of the Masters. Close inspection of the false caps would reveal they were not properly fused to the skull.
TV series[]
The caps in the TV series differed in their function to some extent from the ones used in the novels. It is unclear if a cap can be removed, and what the consequences would be, but it is implied that once installed, they are permanent.
While the Capping made people obedient to the Tripods in the TV show as well, it showed that several characters were able to act malicious and vile towards others, like Duc de Sarlat when he removed Eloise from Will. Black Guards were also shown treating other persons harshly and brutalized them apparently for their own pleasure.
The Vagrants, whose cappings went wrong, were also portrayed differently in the TV series than in the books. Vagrants were seen to form groups, while in the novels they were just lonely wanderers.