The American National Standards Institute (ANSI) established a standard series of paper sizes beginning with the Letter (8.5" x 11") format and progressing to bigger sizes with the designations A, B, C, D, and E. These ANSI standard sizes were defined in 1992, which is a long time after the ISO standard sizes had already been established. The United States government formally adopted the letter-size paper as a national standard in 1921, and this was further reinforced in the 1980s. ANSI Common names for ANSI A paper size include "Letter" and "Ledger" or "Tabloid."
In contrast to the ISO standard sizes, which all have the same aspect ratio of 1:root2, the ANSI standard sizes have two different aspect ratios: 1:1.2941 and 1:1.5455, making it more difficult to increase or reduce between the sizes and resulting in larger margins.