The Cotton Shirt is a mystic armor in Final Fantasy XII that boosts the wearer's Defense by 4 and Magick Power by 1. In the Zodiac versions, it also gives 3 MP. It is the weakest mystic armor in the game and basic equipment.
In the original PlayStation 2 version, the Cotton Shirt is sold for 100 gil in Rabanastre, and is found as a treasure in the dry Giza Plains' Warrior's Wash (75% chance to appear, 70% chance for gil, 50% chance the item treasure is the Cotton Shirt without the Diamond Armlet equipped ), and in the Garamsythe Waterway's flooded No. 10 Channel (65% chance to appear, 35% chance for gil, 50% chance the item treasure is the Cotton Shirt).
In the updated Zodiac versions, the Cotton Shirt is sold for 100 gil in Rabanastre and Nalbina Fortress, and is initially equipped on Penelo. It is found as a treasure in Dalmasca Westersand's The Midfault (65% chance to appear, 40% chance for gil, 50% chance the item treasure is the Cotton Shirt without the Diamond Armlet equipped).
The Cotton Shirt is the weakest mystic armor, giving 4 Defense and 1 Magick Power, as well as 3 MP in the Zodiac versions. It falls off in usefulness fast. Mystic armor offer magick-related bonuses that affect all types of magickal actions, from casting black magick to damage an enemy, to casting white magick to heal the party. Magick Power is also a factor when determining damage dealt by katana, staves, and maces.
In the Zodiac versions, White Mage, Red Battlemage, Black Mage, and Bushi can equip it, and Penelo starts with the license for it so she can wear it no matter the job, and joins the party with it already equipped. If the player starts Vaan in any of the aforementioned jobs, the player can already buy the superior Light Woven Shirt from the shops, and so there is no need to purchase the Cotton Shirt. In the original version, the player can still hold off buying the Cotton Shirt, and go buy the superior Light Woven Shirt when Penelo leaves the party, however, in this version Vaan can wear any equipment piece and the HP-boosting light armor and Strength-boosting heavy armor are likely better suited in this early point.