Second Seal is a type of item in Fire Emblem: Awakening. They are used to change the class of a character.
Second Seals change the class of a character between their sets of classes. Each character has three class sets that they can switch between, which, mostly, include one base class and two promoted classes. Whenever a character uses a second seal and they change their class they return to level one, and their stats will be modified based on the growth rates of their current class and their new class.
Characters that are in a base class can use a second seal once they reach level ten. They can then switch between the base and special classes in their class sets.
Characters in promoted classes can use second seals at any level to change between the base and special classes in their class sets. However characters in promoted classes that are level ten or over can use a second seal to change between the base, special, and promoted classes in their class sets.
Characters in special classes like Manakete and Villager, can use second seals much like characters in base classes. However characters in special classes that are at level thirty can re-class to the base classes and promoted classes in their class sets.
However, characters cannot re-class to the class that they are currently in, unless they are at the maximum level for their class (twenty for normal classes, thirty for special classes). If they do so then they will return to level one, but lose no stats.