Two different player stacks: "Bigstack R" and "Lilstack L"
Stacks are a battle mechanic in World of Final Fantasy where characters are placed on top of each other, whether it be on the player or enemy's side.
Description[]
Characters in a stack have combined stats with one another (such as HP, Strength, and Defense) and the stack also inherits any elemental and ailment resistances and weaknesses, and any non-unique abilities from the characters within it.
A stack acts as a single entity; all characters within the stack act simultaneously—whether attacking, casting magic, or using another command—and their turn is finished. When taking damage or recovering HP/AP, the amount is spread to all characters. When afflicted with a status effect, positive or negative, it is applied to all characters. If an undead character appears in an enemy stack containing at least a living one, any healing items used on the stack bypasses the undead status.
A stack can have up to three characters and is arranged by the size of the characters: large "L"-sized characters are on the bottom, M-sized characters are in the middle; and small S-sized characters are on top. Only one of each size can be present in a stack, and the order is fixed. Up to two three-character stacks can be present on the enemy and player's side each. A two-character stack can consist of an S and M-sized character, an S and L-sized character, or an M and L character. The enemy side can have up to three of such stacks, while the player can have up to two.
Arranging Reynn's "Lilstack R"
Outside of battle, the player can create their own stacks for Lann and Reynn by accessing the Stack Setup menu in the main menu. Depending on whether a sibling appears as a Jiant or Lilikin on the field, the player can create stacks for each form. Jiant forms are L-sized, and the name of the stacks are called "Bigstack L" and "Bigstack R" (for Lann and Reynn's stacks, respectively). Lilikin forms are M-sized, the name of the stacks are called "Lilstack L" and "Lilstack R". Player stacks can be renamed from the menu.
The player's characters always start each battle stacked (with the exception of the tutorial battle in the game's prologue). Stacked characters have the advantage over unstacked characters with their combined stats and elemental resistances, and being able to use any abilities that cost more than 4 AP. Stacked characters, however, also share damage taken, elemental weaknesses, any afflicted status effects, and can only use one turn. The player's stacks can use the Unstack command to unstack itself and each character from that stack can act individually. While each character is weaker when unstacked and has a maximum AP of 4, however, it gives the player the advantage of having more than two turns, and damage and status effects taken is spread out. Unstacked characters can use the Stack command to reassemble their stack.
Stack stability[]
When a stack takes enough hits or when hit with attacks with enough topple strength, a yellow bar with an icon appears over it that indicates losing its stability. When a stack becomes less stable with more attacks, the bar turns red and eventually the characters in the stack begin to wobble. When the stack is hit again with further attacks in this state, the icon flashes and breaks, the stack topples, and all characters in the stack are considered unstacked and become temporarily stunned. A player stack can use the Defend command or manually unstack itself to prevent toppling, and can also use the Wobblestopper item or cast 
Balance or Balancega to restore its stability. A stack always unstacks when its total HP goes to 0.
Stack Abilities[]
The player's stacks can utilize combined abilities depending on the characters within their stacks and the abilities they use. A stack may have access to stronger magic when possessing like-spells (known as Layered Abilities), and may also have access to Combo Abilities, where characters have like-weapons; and Super Abilities, that involve L-sized characters.
Enemy stacks[]
Enemy stack, a "Right Claw Stack"
Enemy stacks are introduced during Chapter 3, in The Nether Nebula. The name of enemy stacks are derived from the Mirages at the very bottom, then "Stack" (i.e. if a stack consists of a Mini Golem and Copper Gnome, the stack is called a "Copper Gnome Stack").