Jolly Roger

The Jolly Roger (海賊旗, Kaizoku-ki or Dokuro) is the traditional flag of European and American pirates, envisioned today as a skull over crossed thigh bones on a black field. Various versions of the Jolly Roger existed over the course of the centuries, for example Calico Jack's Jolly Roger replaced the thigh bones with swords (cutlasses).

Oda himself provided some info on the Jolly Roger, including three possible theorized origins that have come to be accepted by experts:


 * Some say it comes from the French term "Jolie Rouge" or "Lovely red" possibly referring to blood.
 * "Roger" may have come from the word "Rogue" meaning thief or villain.
 * It may have come from the term "Old Roger" an old fashioned name for the Devil.

The Jolly Roger is hugely recognized to mean "danger" worldwide. The skull and crossbones themselves are commonly used on to warn people about dangerous substances or places on signs and bottles.

One Piece
In One Piece, there are many Jolly Roger flags. Jolly Rogers generally help pirates determine who belongs to whose crew, or even who is a pirate in the first place. Many choose to bare their crews flag somewhere on their personal being (such as Portgas D. Ace). Certain nations who understand the ideals of the Jolly Roger or have turned against ideas of the World Government also raise the flag such as the emancipated Drum Kingdom after the fall of Wapol.

However, raising the Jolly Roger is considered a huge crime by the Marines, and anyone seen doing so (may it be fun or for real) will be considered criminals. The designs of the Jolly Roger usually reflect the interest of Captain of the crew. Other times it may be physical resemblance (Whitebeard's mustache or Shank's 3 scars on one eye), a personal item they wear (Luffy's straw hat), or the symbols behind them.

Ideals of the Flag
All those that sail under a Jolly Roger are supposed to show respect for their chosen flag and honour it always as it represents the crew itself. Once the flag is gone, the crew can no longer sail under it and therefore everything linked with the flag is gone. Luffy uses this in making his choice in the Davy Back Fight and takes the Jolly Roger belonging to Foxy instead of a member of the crew, thus preventing ever sailing under it again. He then gives them a new one he drew (badly) himself.

Different crews react differently to these ideals. For example, Wapol did not see what was so important about the pirate flag in comparison to Luffy. Donquixote Doflamingo does not like the idea of anyone tarnishing his flag's name and punished Bellamy for losing under it.

Smileys
Following the introduction of Donquixote Doflamingo, a different type of Jolly Roger has appeared. Rather than the traditional skull and crossbones, the flag is a smiley of a grinning person. The significance of this type of Jolly Roger has yet to be revealed. Yet in some cases, such as Doflamingo's flag and the Human Auction House the smiley has an "against" sign across them. According to Donquixote Doflamingo, the smiley symbol is a growing trend. While most of the smiling Jolly Rogers are connected to Doflamingo, the Heart Pirates are another significant crew with one.

Another smiley has been seen on Whitebeard's life support equipment. Ace also wore two smileys on his hat, one of which was sad as opposed to happy, which was mirrored in his death. Yet another smiley appears on Trafalgar Law's shirt as his Jolly Roger. Kid also possesses a smiley style of Jolly Roger with a stitched-up mouth.

Trivia

 * By defination because pirates were rogues they flew a flag of their own for various reasons, the flag was also significant as offically, they belonged to no country but their own ruling.