A4
A4 is a standard paper size, defined by the international standard ISO 216. It is the normal size of paper for both domestic and business purposes in all countries except the United States and Canada. See paper size for other paper sizes.
It is 210 x 297 mm.
It is sometimes claimed that the aspect ratio of this paper format corresponds to the golden mean (1.618, approximately). This is an error; in fact the aspect ratio for all international standard paper sizes is defined as the square root of two, which ensures that if the paper is folded in half, the halves are the next standard size down:
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The A4 road in England runs from London to Bristol.
The A4 highway in France runs from Paris to Strasbourg. It's a part of the European routes E50 and E25.
The A4 class Mallard holds the world speed record for a steam locomotive.
A4 is also the actual model number for the German V-2 rocket.
A4 is a model of an Audi car, see Audi A4.