To trigger these changes, a character must target their desired arte within what are called Fields of Fonons, alternatively known as FOF Circles, which take on the forms of ring-shaped auras on the ground. There are six possible FOF Circles, one for each element: Earth, Fire, Wind, Water, Light, and Dark. FOF Circles only last for only about five seconds, so for spells that require longer than that to cast, the FOFs will have to be carefully created. Enemies, both boss and generic, are capable of performing FOF Changes as well.
FOF Circles have three levels of strength to them: 25%, 50%, and 100%. FOF Changes are generally only possible with 100%-complete Fields of Fonons. Artes that leave behind 25% FOF Circles must be performed four times to make a complete FOF; likewise, artes that provide 50% FOF areas must be performed twice. Artes that create 100% FOF areas require no further uses for the full effect. Some enemies are capable of creating FOFs with their normal attacks. Incomplete FOFs have thin, white auras, whereas complete FOFs are the color of their element. The color for each element is as follows:
Dark FOF Circles bear the properties of both Earth and Water FOF Circles, and Light FOF Circles bear the properties of both Wind and Fire. There are no artes that specifically require a Dark or Light Circle. Artes can only be powered up by specific FOF Circles, and exactly which ones are necessary for which artes is not stated in the game. When an FOF Change is performed properly, the FOF Circle is absorbed by the performing character, but it is possible for more than one character to alter their artes in the same FOF Circle, as long as they are performed at approximately the same time. FOF Changes involving fonic artes will absorb the field at the beginning of casting.
Furthermore, Tear Grants is the only character who has spells specifically for creating FOF Circles. While other characters can create their own FOF Circles, melee characters like Luke fon Fabre or Guy Cecil can leave behind only 25% complete circles using certain elemental strike artes. Jade Curtiss and Anise Tatlin can also leave FOF Circles, with higher percentage charges created by their more potent spells, such as all of Jade's advanced spells which create 100% complete FOF Circles. Only one arte in Natalia Luzu Kimlasca-Lanvaldear's repertoire, Revive, is capable of creating an FOF Circle. Tear, in contrast, can create FOF Circles at 50% completion with minimal spellcasting time. She can cast one of these spells twice in order to complete the circle; however, casting a different arte will end the chain. Tear can stack her "Invoke" arte onto a different character's leftover FOF Circle.
Finally, any arte with an FOF Change can be used in its corresponding FOF Circle, even an incomplete one, if it is equipped with a "Sunlight Chamber", after the Fon Slot Chamber system is enabled upon returning to Baticul. The Sunlight Chamber, however, only allows FOF Changes to occur within the incomplete circles for its particular element. An arte equipped with a Sunlight Chamber can not use an incomplete Light or Dark FOF Circle for an FOF Change.