A headjack is the small data port created on synthetically-grown humans who become connected to the neural-interactive virtual reality known as the Matrix. The headjacks were located at the base of the skull, just above where the neck meets the back of the head. Approximately 1 inch (2.54 cm) in diameter, it provided room within the otherwise close quarters of the human cranium for a data probe approximately 8 inches (20 cm) in length and 0.79 inches (2 cm) in diameter. These were installed during the synthetic humans' fetal stages and altogether redirected the brain's signals into any data probe plugged into the headjack. The headjack was the interface used to primarily connect to the Matrix. More than just connecting to the Matrix, however, the headjack was also used by redpills to connect to any compatible simulated reality like the Construct. Once a redpill was connected to either the Matrix or the Construct, the headjack could then be used to upload data directly into the human brain. This was used by the Resistance to great effect, uploading instructional information to support agents during training and even during mission proper. Some examples of the upload-able information used by the operatives were gun handling, a range of self defense and martial arts techniques, and helicopter piloting. The manner by which the headjack and data probe worked had not been sufficiently described. But communication by the headjack was probably a mixture of electrical and electromagnetic transmissions, where the headjack was the central interface, with perhaps thousands of tiny connectors or electrodes that meld into and throughout the synthetic brain, or woven through the central nervous system at the brain stem.