Dials are strange, mysterious devices made from the remains of particular shellfish, which have the ability to store energy and matter. The shellfish remains are gathered from the shallows near the shore of the White-White Sea. Though they originate from Skypiea, some of them can reach and be used in the lower seas.
There are many types of Dials that store various things depending on their type and most of them seem to be common throughout sky islands, though their rarity can vary by type with some being native to specific regions and some extinct. Dials come in many sizes and shapes, from the size of an average seashell to the size of a human. Dials store energy and matter itself, usually by that particular type of energy or matter being absorbed by a tiny hole or holes on one side of the Dial. The content can then be released by pressing the apex on the other side of the Dial. As long as they are not used or damaged, Dials seem to be capable of storing their contents indefinitely. If the shell of the Dial is broken they can become unusable.
Dials based on clouds are rendered useless on the Blue Sea due to the inability of the clouds to survive at that atmospheric temperature and pressure.
Dials are capable of letting the energy out in different amounts. Some use their contents all in one blast, such as Impact Dials, Axe Dials, etc., while others are capable of storing multiple uses within their contents until it runs out, such as the Flame Dial, which then has to be refilled with that content for the dial to be used again.
On sky islands, battles are fought differently than down in the Blue Sea. Oftentimes, weapons are upgraded, or even combined, with Dials. This creates incredibly powerful weapons and combat styles, allowing for tactics that would otherwise be unachievable.
Impact Dials, as well as Reject Dials and Axe Dials, are generally hidden under bandages or a glove during Sky Warfare. Sometimes even Breath Dials, filled with noxious gases or bad smells, will be hidden as well. This allows for sneak attacks as the opponent will likely not be able to tell what kind of Dial it is, or that there even is one, until it's actually used.
Dials can provide even more of an advantage against those who do not know what they are and thus do not expect their opponents to be able to do such things. People who do not know about dials, such as many residents of the Blue Sea, can even mistake the power of a Dial for an innate ability of the user, such as when Perona mistook a blast from Usopp's Impact Dial as superhuman strength, since Dials are non-threatening in appearance, often small, and rare on the Blue Sea.
When using Dials in combat, the user will normally say the name of the Dial but omit the "Dial" part, such as "Impact!"
The Axe Dial (斬撃貝,Akkusu Daiaru?, literally translated as "Slicing Attack Shell") is a rare dial not found in Skypiea. It delivers a thin blast of air in the shape of an X that is capable of slicing through iron in one blow. The fifty Goat-like Enforcers, known as the Militia, each have one, and their commander, Yama, has a belt of them. Like the Impact Dial, its main use is in Sky Warfare.
The Ball Dial (気泡貝,Bōru Daiaru?, literally meaning "Air Bubble Shell") releases a puff of clouds that forms into a ball shape upon being released, which can be used for sitting or standing on, or be applied with explosives and other features, as proven by Satori.
The Breath Dial (風貝,Buresu Daiaru?, literally translated as "Wind Shell") is capable of storing air-currents. These Dials are usually used to propel wavers through water and on ships if there is no wind for sailing. These Dials can also be attached to the sides of a snowboard or "skates" as alternative methods of travel, as demonstrated by the Shandia.
Unlike Jet Dials, Breath Dials can carry smells and aromas as well. They are handy for storing gases, such as poisons, bad scents, and flammable substances.
The Flame Dial (炎貝,Fureimu Daiaru?, literally translated as "Blaze Shell") absorbs and releases fire; it is not to be confused with Heat Dials.Shura's giant bird, Fuza, has one in its mouth, which enables it to breathe fire. Flame Dials can also be used in sky warfare, concealed in the user's hand beneath a glove or bandage to allow them to burn their opponents.
The Flash Dial (閃光貝,Furasshu Daiaru?, literally translated as "Flash Shell") is one of the three known types of Dials that store and release light. The Flash Dial lets out an extremely bright amount of light for a very brief period, effectively causing temporary blindness if used against a person similar to a flash bang grenade. It is likely first seen in Braham's Flash Guns, but was first seen in its raw form when Usopp used one in his fight against Luffy.
The Heat Dial (熱貝,Hīto Daiaru?, literally translated as "Heat Shell") stores and releases heat energy. It is commonly used to power up ovens and other heating devices for citizens of sky islands. It appears to be able to let out heat in different amounts. It can also be used in sky warfare, as seen with Shura's Heat Javelin.
The Impact Dial (衝撃貝,Inpakuto Daiaru?, literally translated as "Impact Shell") may be hit by anything blunt or incapable of slicing (although it was shown absorbing axe and sword strikes in the Non-CanonG8 arc) and will absorb the kinetic energy. Once the apex of the Dial is pressed, the stored energy is released, usually causing internal injuries to a living body, while not leaving any visible external damage. This blunt force is enough to wound even Luffy, who is usually immune to blunt force due to the Gomu Gomu no Mi. The Impact Dial can also absorb the impact of explosions.
The Impact Dial, while not as dangerous as the Reject Dial, can still hurt the user through the backlash caused when such a large amount of energy is released, as shown with the most frequent user, Usopp, and Nami as well during her fight against Kotori. However, many Sky Warriors have been shown to be highly resistant to this recoil. It's unknown if there is a limit to the energy it can absorb, but so far these Dials have been able to absorb all the impact energy used against them.
The Jet Dial (噴風貝,Jetto Daiaru?, literally translated as "Wind Jet Shell") is a more powerful version of the Breath Dial, which blows out a burst of air at great speed. This allows them to be used for faster propelling purposes. They were once used in wavers when they were common, but were replaced by Breath Dials when they became extinct. Nami's Waver has a Jet Dial, making it faster than a normal one. Enel's warship, the Ark Maxim, has a backup propulsion system powered by two hundred Jet Dials.
They can also be used in battle: Gedatsu wears a Jet Dial on each of his arms to increase both the speed and power of his punches. The punch is fast enough to cause his fist to seemingly disappear, and strong enough to injure Chopper while in his Guard Point. However, Gedatsu dislikes using this technique due to the Jet Dial's friction tearing up his clothing.
The Lamp Dial (灯貝,Ranpu Daiaru?, literally translated as "Light Seashell") is similar to the Flash Dial, capable of storing and releasing light. Unlike the Flash Dial, however, it releases light steadily rather than all at once, hence the name, making it much less useful for combat purposes, but much more useful for domestic purposes. It was first seen during Pagaya and Conis' explanation of the various dials.
The Eisen Dial (鉄貝,Aizen Daiaru?, literally meaning "Iron Shell") is extremely rare, and one of the most formidable dials used for combat. It emits a stream of "iron cloud" that can form into different shapes, like a cloud, but is as hard as iron. It can lengthen and/or widen without limits and can even form thick defensive walls. Ohm possesses an Eisen Dial, inside the pommel of his sword. Eisen is German for "iron".
The Milky Dial (雲貝,Mirukī Daiaru?, literally meaning "Cloud Shell") can store clouds. This can be used to create pathways in the air out of clouds, upon which devices capable of moving upon clouds can travel. Gedatsu also installed Milky Dials in the soles of his shoes allowing him to fly, protecting him from harm in his Ordeal of Swamp.
The Reject Dial (排撃貝,Rijekuto Daiaru?, literally meaning "Rejection Shell") is a much rarer and more powerful version of the Impact Dial. When used, it unleashes whatever force it had absorbed up to that point tenfold, but the backlash is hazardous to the user, powerful enough to destroy the body of an inexperienced user. Due to this, they are rarely used. The ShandiaWyper owns one, and used it three times during the Skypiea Arc; he seemed largely unaffected the first time he used it, the second time exhausted him, and the third time nearly killed him.
The Tone Dial (音貝,Tōn Daiaru?, literally translated as "Sound Shell"), often abbreviated as TD in the Blue Sea, is able to record and replay sounds spoken into it, akin to an audio recorder. It was first shown to Luffy and Usopp by Pagaya and Conis, when they explained the various functions of Dials.
The most prominent use of a Tone Dial in the series comes from Brook, who bought one from a merchant on the Blue Seas, and used it to record the last song played by the Rumbar Pirates for their good friend, Laboon; he stores the Dial inside his skull.
During Brook's career as a world-famous rock star, he sold many Tone Dials with his music recorded on them. Given how many he sold, stated by his manager to be in the millions, they seem to be the most common of the Dials to make it down to the Blue Seas. Oda explained in an SBS that during the timeskip, Tone Dials became mainstream in the Blue Sea. They are sold in pairs and have been modified so they can be inserted into headphones and on special turntables so that people can enjoy music portably. The modern uses for Tone Dials were shown in the anime prior to the SBS.
The Vision Dial (映像貝,Bijon Daiaru?, literally translated as "Image Seashell") is used for capturing images and playing them back, comparable to a modern-day camera. It is useful for scouting and recording evidence.
The Water Dial (水貝,Wōtā Daiaru?, literally translated as "Water Seashell") is used for storing and releasing water. It was spoken of as being capable of extinguishing the large fire left after Enel's judgment, but was not seen in use.
The Thunder Dial (雷貝,Sandā Daiaru?, literally translated as "Lightning Seashell") is used for storing and releasing electricity. It is used in the Ark Maxim's engines. It is only named in the Unlimited Cruise games.
The following events are Non-Canon and therefore not considered part of the Canon story.
The Mad Dial (狂乱貝,Maddo Daiaru?, literally meaning "Frenzy Seashell"), in the video game One Piece: Pirate Warriors 2, is a special type of dial appears in a laboratory on Punk Hazard. It releases a dark purple fog that infects the mind of whoever inhales it, driving them insane, hostile, and aggressive. The victims gain a dark, cloud-like aura around them, and their eyes turn red. One interesting side effect of this dial is that it can alter the personality of certain individuals, such as causing Usopp to lose his general negativity, thus rendering himself vulnerable to the Negative Hollows generated by the Horo Horo no Mi, though repeat Negative Hollow attacks by Perona eventually restored his negative personality, curing him of the dials effects. Even though the dials can alter a person's personality, people with high doctrine will retain their principles such as Sanji. Despite being affected by a Mad Dial, Sanji is still unable to kick any woman due to his strong principle to not kick any woman such as Nico Robin even if it means his death. This type is known as Frenzy Dial in the game's English localization and was previously named the Fog Dial (霧の貝,Kiri no Daiaru?) by Smoker.
The Control Dial (操作貝,Contorōru Daiaru?, literally meaning "Management Seashell"), in One Piece: Pirate Warriors 2, appears in a laboratory on Punk Hazard. It creates a strange circular ray, similar to Foxy's Noro Noro Beam and is able to control the people affected by the Mad Dials' fog. It was previously named as Trembling Dial (震える貝,Furueru Daiaru?) by Smoker.
Usopp traded rubber bands in Skypiea for Dials, touting them as the power that beat Enel. As rubber is not found in Skypiea, it is a mysterious substance to the Skypieans.
It is possible to get a hold of a dial without going up to the sky. The Rumbar Pirates bought a Tone Dial from a market, which they used to record their final performance for the whale Laboon, in hopes that their crewmate Brook would meet him again. Dial-based equipment, such as Wavers, also occasionally make their way down to the Blue Sea. It is also revealed that somehow during the two-year time skip, Brook sold about two million Tone Dials with his music recorded on them, similar to a CD or cassette tape in the real world.
In Strong World, Shiki used a Tone Dial to communicate with the Straw Hat Pirates and Nami used it to leave the last message to her crew. Although the possibility was mentioned, it is not clear if Shiki actually acquired it on a Sky Island itself or on the Blue Sea as the Rumbar Pirates did.
The non-canon Mad Dials and Control Dials from the video game One Piece: Pirate Warriors 2 ability to corrupt and control various characters is similar to the Dark Magic that appears in the Dragon Ball video game, Dragon Ball: Xenoverse. Both are capable of turning good characters into evil versions of themselves that can be manipulated/controlled by the main antagonists of their respective games. Also, any characters affected possess traits in common such as a dark aura, red eyes, and more aggressive and/or violent tendencies. Both also cause the main protagonists' allies/comrades to turn against them, forcing the protagonists (and their uncorrupted allies) to fight them in order to restore them to back to their normal selves. Both also play a prominent role in each of their games' overall plot.