A4 paper size is 210mm × 297mm, many people have to ask why is called A4, why set this size is 210mm × 297mm, why is the unit is mm for the unit, A4 paper is used the most frequent among printing paper. We can experiment with a series a sheet of paper, folded in half on one side of the length, and the ratio of the length and width of the two sheets folded out is still the same as the original paper. In this case, we will reduce the image on A3 paper to A4 paper, and if the ratio is the same, then we can reduce the image on A3 paper to A4 paper in the same proportion. The picture will not be deformed, this is what the Lehitian Berger ratio is called.

Do you know the size of A4 paper that we often use for printing or copying? For 210mm × 297 mm. Why this specification? Take another look at the size of A3 paper. 297 mm × 420 mm. You can try and place two A4 sheets of paper, long edges against long edges, and place them side by side. Compared with a sheet of A3 paper, you will find that the size is exactly the same.