The United States is using a different paper size system compare to the rest of the world. They have different sizes standard sizes. The US Letter paper size is the most popular size used throughout the US. It is used in Canada and Philippines also.
Letter, Legal, Ledger and Tabloid are most commonly used for everyday activities. In 1996, the American National Standards Institute adopted ANSI/ASME Y14.1 which defined a regular series of paper sizes based upon the de facto standard 8 1⁄2 in × 11 in (216 mm × 279 mm) Letter size which it assigned "ANSI A", intended for technical drawings, hence sometimes labeled "Engineering". This series is somewhat similar to the ISO standard in that cutting a sheet in half would produce two sheets of the next smaller size and therefore also includes Ledger/Tabloid as "ANSI B".
The ARCH series of paper sizes is defined in the ANSI/ASME Y14.1 standard. The ARCH sizes are commonly used by architects for their large format drawings. The ARCH series preferred by North American architects instead of ANSI because the aspect ratios (4:3 and 3:2) are ratios of small integers. Furthermore, the aspect ratio 4:3 matches the traditional aspect ratio for computer displays.