The us Series
The United States, Canada and Mexico use a different system of paper sizes compared to the rest of the world. The current standard sizes are unique to that continent, although due to the size of the North American market and proliferation of both software and printing hardware from the region, other parts of the world have become increasingly familiar with these sizes (though not necessarily the paper itself).
The North American paper sizes don’t rely on a logical system like the ISO standard does. This makes paper sizes infrequent in size gradation. The US Letter size is the most commonly used throughout the USA, and is also regularly used in the Philippines and Chile.
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 8.5 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'. Unlike the ISO standard, however, the arbitrary base sides forces this series to have two alternating aspect ratios. For example, ANSI A is less elongated than A4, while ANSI B is more elongated than A3.
There is a corresponding series of paper sizes used for architectural purposes defined in the same standard, ANSI/ASME Y14.1, which is usually abbreviated 'Arch'. This series also shares the property that bisecting each size produces two of the size below, with alternating aspect ratios. It may be preferred by North American architects because the aspect ratios (4:3 and 3:2) are ratios of small integers, unlike their ANSI (or ISO) counterparts. Furthermore, the aspect ratio 4:3 matches the traditional aspect ratio for computer displays.
The size Arch E1 has a different aspect ratio because it derives from adding 6 inches to each side of Arch D or subtracting the same amount from Arch E. An intermediate size between Arch C and D with a long side of 30 inches (760 mm) does not exist.