Sakazuki (Ceremony)

The sakazuki (or "sake cups"), refers to a formal ceremonial process in which two or more individuals form a bond akin to that of family in an exchange of sake cups.

Overview
Pirates appear to perform sakazukis when establishing a fleet. It is also to be used by the yakuza of Wano Country.

Three types of sakazuki have been featured so far:
 * Brothers' Sake Cups (兄弟盃) - used during a formation of a brother-brother relationship.
 * Parent-Child Sake Cups (親子の盃) - used during a formation of a parent-child relationship.
 * Followers' Sake Cups (子分盃) - used during a one-sided pledge of loyalty.

Brothers' Sake Cups are performed between equals, signifying a relationship like that of brothers. Parent-Child Sake Cups are usually performed between a superior and their subordinate, to signify a parent and child relationship.

After all individuals involved have sake poured in their respective cups (with the largest cup in the possession of the leader in the event of Parent-Child Sake Cups) they make an oath or perform a toast before drinking from the cup, swearing themselves to each other. It also seems that the individuals get to keep their respective cups.

Brothers' Sake Cups

 * Monkey D. Luffy, Portgas D. Ace, and Sabo - In order to reinforce their bond, Ace, having heard that in order to become brothers they only needed to share some sake, stole some from his foster mother, Dadan, and together with Sabo and Luffy performed an informal Kyoudai-Sakazuki.

Parent-Child Sake Cups

 * Golden Lion Pirates Fleet - In One Piece Film: Strong World, Shiki attempted to form a pirate fleet with approximately 50 other pirate crew, but before they could complete the Oyako-Sakazuki, they were interrupted by a raid from the Straw Hat Pirates.
 * Big Mom Pirates and Jinbe - When Jinbe affiliated himself with the Big Mom Pirates, he apparently performed an Oyako-Sakazuki, and kept the cup. This is indicated when he officially resigned from the Big Mom Pirates and offered his sakazuki as a gesture.
 * Kyoshiro Family - When new yakuza join the family, it is implied that they are inducted with the sakazuki ceremony.
 * Whitebeard Pirates and Portgas D. Ace - In the fourth chapter of One Piece episode A, Ace formally becomes Edward Newgate's son with an Oyako-Sakazuki.

Followers' Sake Cups

 * Straw Hat Grand Fleet - After defeating the Donquixote Pirates, the seven pirate crews made up of the Beautiful Pirates, Barto Club, Happo Navy, Ideo Pirates, Tontatta Pirates, New Giant Warrior Pirates and the Yonta Maria Grand Fleet attempted to swear themselves in as Luffy's subordinate crews. Luffy declined, stating his perspective of the Pirate King, which endears him more towards the crew captains. As such, they one-sidedly swore themselves to him in a Kobun-Sakazuki.

Real World applications
In the real world, sakazuki is an ancient formal ceremony in which individuals become family through an exchange and toasting of sake. The most common application in the present day are wedding ceremonies, when a wife marries into a man's household.

Kyoudai-Sakazuki and Oyako-Sakazuki are frequently used by Japanese underworld organizations' referred to as yakuza.