If you look at Sino-Japanese text printed in the Chinese Song or Japanese Mincho typeface (similar to serif typefaces in European languages), you’ll notice that the horizontal strokes in characters are much thinner than the vertical strokes. Here’s why:
The printing press appeared in China during the Song Dynasty. At the time, each print block contained two portrait-oriented pages placed side by side. The print blocks were all cut from rectangular planks such that the wood grain ran horizontally. Because the grain ran horizontally, it was fairly easy to carve patterns with the grain, like horizontal strokes. However, carving vertical or slanted patterns was difficult because those patterns intersect with the grain and very easily break. This resulted in a typeface that has thin horizontal strokes and thick vertical strokes. To prevent wear and tear, the ending of horizontal strokes are also thickened. These design forces resulted in the current Song typeface.