This paper shape ~ and its system of sizes ~ has been adopted throughout most of the World. In most countries the ISO range has replaced the traditional selection of papers for everyday use. The reason for this popularity lies behind the advantages of the paper's shape. Way back in the eighteenth century this shape was described ~ and slowly adopted ~ in France and Germany. In 1975 the shape became formalized by the International Standards Organization ~ ISO#216 ~ as also did the range of sizes.
The shape is such that when a sheet is halved the proportions remain the same. This property is unique to the chosen shape. When a square shape is halved then a thinner rectangular shape is formed. There is intersting maths behind this ~ dealt with elsewhere.
It is true that a portrait sheet when halved becomes a landscape sheet ~ but a 90degree turn sorts that out. This unique maintenance of proportion has numerous advantages such that the ~ almost ~ world wide adoption was obvious and sensible to ~ almost ~ to all. I have a separate note on this [113024].
Since the continental countries ~ which were at the forefront of the moves to standardize the shape ~ used the metric system, it was natural for them to choose a metric size for the standard. The metre was their chosen standard for length, so the standard for area is one square metre. This is not the same as one metre squared ~ on which I have a note [113036]. The United States ~ for example ~ has not standardized on the metric system and so has been slow to adopt ISO216. There is no compulsion to do so ~ and they are not alone [113043] .