Background
The original Thieves Guilds were established as a way for the Uzumaki Clan to come together after the destruction of Uzushiogakure, that's why they are only accessible by a Seal Array. It was used, not only as a meeting place, but also a place where resources, such as money and food could be shared between them until they could establish themselves in their new home. Over time, they saw people not of their clan, who were suffering the same problems that they had had and took them under their wings, giving them training when requested, along with a temporary place to call 'home'.
Modern Day Thieves Guilds
Each 'big' city has a thieves' guild, identifiable by a bas-relief depicting a bird inside of a moon shining over a depiction of the city that it is in. Members of distant guilds can go to another guild for succour or information. While not strictly necessary, most guild members join the guild that is in their hometown. Over the years many ninja spymasters have tried to infiltrate the network of thieves' guilds, due to the ability to bypass having to build their own information networks, but all have failed. Conversely, many Thieves Guild Leaders have tried to get a step in in some of the greater ninja villages, but have always been caught before they could get fully established.
Identification
While not strictly necessary while on business, or even when just checking into a foreign guild, there is a type of uniform, consisting of some sort of jacket (or coat, vest, etc…) that the member is used to wearing. They are also given a Venetian mask that has a golden floral leaf design embroidered onto it, which indicates both what division of the guild as well as their rank within that division that they operate in. It is not necessary, but wearing them tends to clear the way and reduce suspicion when meeting foreign guild members for the first time, though the guild leader and their advisors tend to wear theirs all the time.
Blue: Guild Leader
Purple: Advisor
Silver: Information Gatherer / Disseminator - has silver embroidery
Black: Assassin
Red: Apprentice Assassin
White: Thief
Yellow: Apprentice Thief
Green: Informant
Any: Bird Keepers
Upon joining a guild, members are given a bird that is the same as the one that is on the bas-relief above the door of the guildhall. Guild members hand their bird over to a foreign guild when entering the city, telling them that there is a member from another guild in the city and to leave them alone if they are seen on business. Members of the thieves' guild can be a part of multiple divisions, and are given a mask matching that division, and another bird reflecting where they joined that division.
Land of Wave: Tui
Land of Mountains: Sage Thrasher
Code of Conduct
Strange as it seems, the members of the assassin division of a Thieves' Guild tend to have their own code of conduct that they operate by, though the thieves will do whatever it takes to take down their mark. The local guild leader will do whatever is necessary to protect the guild, even if it means taking down their best members. However the advisors tend to kill off potential successors so that they can remain in power longer. This also happens to be why advisors traditionally don't ascend to the position of guild leader. Captured full-fledged assassins and thieves are executed without prejudice due to the fact that they tend to have advanced working knowledge of the thieves guild system.
Full-fledged division members are encouraged to take on apprentices, though it is not required if it would impact on their normal lives. Some full-fledged division members take an apprentice on one of their jobs, but do not take them on full-time. At times a team of full-fledged members are needed to take on an assignment, rare as it is.
The different Divisions of the Guild have their own mottoes, which also serve as creeds. The creeds serve as a descriptor of the work that is done by the members of those divisions.
Thieves Division: Laissez-nous faire(Let us do what we want to do)
Assassins Division: Qu'il soit fait(Let it be done)
Information Gathering Division: Faire connaître (Make Known)
Bird Keepers: Apte à voler(Fit to fly)
Leadership: Méfiez-vous des autrui(Watch out for the others)
Locations
Normally Guild Locations are kept within sewer systems, caverns or some other underground area. However, the Guild in the Land of Waves is built on the ruins of Uzushiogakure. It also happens to be the birthplace of the system of the Thieves' Guilds.
Land of Wave Faction
After Naruto's defeat of Tenkatagi Tatsuo, the faction in the Land of Wave is now defunct. It was replaced by Neo-Uzushiogakure with Zabuza as its leader. Naruto donated Katon: Karyū Endan, Suiton: Mizurappa(Wild Water Wave) and Fūton: Ressenpū. He also gave them a notebook containing every seal character that he knew at the time, and some theories on how they work together, however he only included the designs of the basic storage seal and generic explosive tags. He convinced Amayah to part with some jutsu of her own. She chose to leave behind the Suiton: Mizurappa(Water Trumpet). She would have left a futon: Ressenpū, but Naruto had already done that. They both also recorded how to combine the elements for Shakuton, Hyōton and Futton, though they stopped short of giving them any technique scrolls.
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A/N
This will be updated as needed.