The 3-1-1 rule is the standard TSA regulation for carrying liquids, aerosols, gels, creams, and pastes in your carry-on luggage through airport security. The numbers serve as a mnemonic: 3 stands for the maximum size of each individual container, which must be 3.4 ounces (100 milliliters) or less. The first 1 indicates that all these containers must fit comfortably into one quart-sized, clear, resealable plastic bag. The final 1 represents the limit of one bag per passenger. The requirement for a clear bag is specifically so that security officers can easily identify the contents without opening the bag. Common items that must follow this rule include toothpaste, shampoo, lotion, and perfume. There are important exemptions to the 3-1-1 rule for medically necessary liquids (like insulin), breast milk, and baby formula/juice, but these must be declared to the TSA officer for separate screening. In 2026, while some airports are installing new 3D scanners that may eventually phase this out, the 3-1-1 rule remains the active and primary protocol for most travelers worldwide.