Spider-Man's web-shooters were perhaps his most distinguishing trait, after his costume. Peter had reasoned that a spider (even a human one) needed a web. Since the radioactive spider-bite did not initially grant him the power to spin webs, he had instead found a way to produce them artificially. The wrist-mounted devices fire an adhesive "webbing" (see below) through a threaded adjustable nozzle. The trigger rests high in the palm and requires a double tap from the middle two fingers to activate, eliminating the chance of accidental discharge when forming a fist.[54] To accomplish this his hands are often in a distinctive hand position when he fires them (resembling the sign for "I love you" in American Sign Language, reverting someone giving you the "Evil Eye" in Italian superstition or the horned hand heavy-metal gesture). In order to fire the webbing, Spider-Man's fingers must hit the sensor precisely.
Spider-Man must steadily replenish his webbing supply, reloading his web-shooters with small cartridges of web fluid, which is stored under high pressure. In early stories, he carries his extra supplies in a utility belt worn under his costume.[55] Later on, he equips the web-shooters with a bracelet-like carousel that automatically rotates a new cartridge into position as he empties them. When in use, a steel nipple in the carousel pierces the seal of the cartridge, and allows the fluid to travel through an air-tight channel toward the nozzle. Pressing down on the palm-trigger of the web-shooter causes the valve in the nozzle to open wider, expelling the fluid. Releasing the trigger causes the valves to close, cutting off the web-line or fluid. If Spider-Man creates any variation to his normal web formula that's too strong for the pinch valves to sever, he might end up being tangled up or tied to the object he attached his web to. This has happened on more than one occasion. His web-shooters require constant maintenance and on more than one occasion suffer jams or malfunctions.