This web content discusses the difference between A4 and Letter paper sizes and provides information on various other paper sizes. The author explains the two predominant paper size systems in use around the world – the international system (ISO 216) and the North American system. The ISO paper sizes, particularly the A series, are widely used and have an aspect ratio equal to the square root of two, making them easy to enlarge or reduce for printing. The article also mentions the B and C series, as well as North American paper sizes like Letter and Legal. Tips for saving time and printing A4 documents on Letter and vice versa are provided. The content also discusses untrimmed paper formats (RA and SRA) commonly used in commercial printing.
https://www.swiftpublisher.com/useful-articles/paper-sizes-and-formats-explained