You might have seen a bamboo mat — for making sushi rolls. These mats are called "Su (簀)" in Japanese and you should see the ones for papermaking — Hiki-Su is unbelievably fine and delicate. 

Su for paper making is called Hiki-Su

Because bamboo has nodes that make bumps, artisans of Hiki-Su use only the culm section (the parts between the nodes) which are limited in length. They carefully join each hyper-thin strip with silk string to make a larger, smoother Hiki-Su that doesn't fall apart after years of the papermaking process. 

The bamboo rods are not actually joined, they are lined up end-to-end with silk string.

Only highly skilled artisans can make Hiki-Su, (also, the silk string has specifically been made for Hiki-Su!) and very few of them are available today. 

Su placed in a frame called Keta in sheet forming process

Paper in the image: CON1451 Kobayashi Kadoide Light, 24g, 25x37" (Heritage Washi)

If you want to know if the washi is high-quality and handmade, look at the surface carefully. The thin marks of Hiki-Su are one of the easiest signs and you will feel the breathtaking work of the artisans who made them.