Ordinary doors, such as classrooms and offices, can be cut in half with A4 paper, and doors that are too big (such as two glass doors) will be cut in half with A3.
Other formats:
Before ISO 2 16 was widely adopted, there were many different paper formats in the world. These formats are not a coherent system, and many of them are not defined in metric units. The format of ISO 2 16 follows the ratio of 1: √ 2; Two pieces of paper put together have the same length-width ratio and side edges. This function simplifies many things, such as reducing and copying two A4 sheets into one A4 sheet.
Enlarge and photocopy A4 paper into A3 paper; Photocopy and enlarge a half A4 paper into an A4 paper, and so on. The main obstacles to this standard are the United States and Canada, which still use the stationery, legal and administrative paper size system. (Canada uses group P paper size, which is actually the paper size used in the United States, and then takes the nearest metric size. )