Editing Preferred number (section)

== Paper documents, envelopes, and drawing pens ==
{{Main|Paper size}}
Standard metric paper sizes use the [[square root]] of two ({{sqrt|2}}) as factors between neighbouring dimensions rounded to the nearest mm ([[Georg Christoph Lichtenberg|Lichtenberg]] series, [[ISO 216]]). An A4 sheet for example has an aspect ratio very close to {{sqrt|2}} and an area very close to 1/16 square metre. An A5 is almost exactly half an A4, and has the same aspect ratio. The {{sqrt|2}} factor also appears between the standard pen thicknesses for technical drawings in ISO 9175-1: 0.13, 0.18, 0.25, 0.35, 0.50, 0.70, 1.00, 1.40, and 2.00&nbsp;mm. This way, the right pen size is available to continue a drawing that has been magnified to a different standard paper size.