Most parts of the United States, Canada, and Mexico use a different standard for their paper sizes.
The American National Standards Institute (ANSI) uses inches as the main unit of measurement. The most common size in this series is ANSI A, otherwise referred to as the Letter size. The other common size is ANSI B, which comes in two different orientations - Tabloid (portrait or vertical) and Ledger (landscape or horizontal).
The ISO alternative to Tabloid and Ledger is called A3. Just like ISO paper sizes, cutting the sheet of the ANSI paper size will produce two sheets of the next smallest size. For example, if you cut the Tabloid or Ledger page in two, you are left with the Letter or ANSI A size document.
The ARCH paper system is used for architectural purposes and other large-format drawings. The whole number ratios of the paper sizes, which are at 4:3 and 3:2, make it more ideal for architects. Similar to ANSI and ISO, cutting these sizes into half also result to two of the next smaller size.