One of the Three Great Dōjutsu (San Daidōjutsu), the Sharingan is the kekkei genkai of the Uchiha clan. Unlike the other Dōjutsu, not all members of the Uchiha Clan possess the Sharingan. This great Dōjutsu seems to awaken only on exceptional Uchiha members and only in times of great stress. Once awaken, the pupil turns red and a single tomoe (巴) seal appears around the center pupil. As the user masters and evolves their eyes, the tomoe (巴) seals increase, topping 3 per eye. The main ability of the Sharingan, like any other Dōjutsu, is to enable the user to see the flow of chakra. This ability is further improved as the Sharingan gives color to chakra, enabling the user to differentiate chakras from different people or from different sources. Although the user can see chakra through matter, he cannot see beyond dense matter or in long distances. Coupled with the ability to see chakra, Sharingan users gain acess to an incredible clarity of perception that enables them to read insanely fast movements or minor details, enabling them the ability to foresee in an easier way, traps, incoming attacks, etc. This clarity of perception is so great that a Sharingan user can sometimes start performing a counter to an attack that is still being performed, enabling the counter to be launched only a split a second after the attack. This clarity of perception leaves Sharingan users able to track high speed movement easily as well as to read through handseals. This last ability enables them to learn techniques much faster and sometimes copy them directly from the enemy, as long as the user has the necessary chakra affinities and requirements to perform it. Finally, the Sharingan is often linked to Genjutsu and mind-manipulating illusions, mostly because the user can, mostly through Eye Contact, directly and precisely use his chakra to manipulate the enemy's chakra flow. In return, the users clarity of perception gives him the ability to more easily decipher visual illusions cast upon him and see past them. The Sharingan has a great cost to the user though. The chakra consumption disables its use for long periods of time and, even when the user grows stronger, it's still hard to keep it active for long periods of time.
