Traditionally, katanas are not straight. The katana is a type of Japanese sword with a curved blade, characterized by its distinctive shape and design. The curvature, known as the "sori," allows for more effective cutting and slicing motions. The curve of the katana also helps to distribute the force of the impact along the length of the blade, making it stronger and less likely to break.

However, it's worth noting that there are variations in the curvature of katanas. Some katanas may have a slight curve, while others can have a more pronounced curve. The degree of curvature can vary based on the specific style, period, and intended use of the sword.

That being said, there are also straight swords in Japanese history, such as the tachi, which predates the katana. The tachi is an earlier form of the Japanese sword that is often straighter compared to the katana. So, while straight swords exist in Japanese sword history, the katana itself is typically known for its curved blade.