DISCLAIMER: The video above was published on YouTube by Trace Study. The video is part of an amazing series of videos featuring prof. Kenji Komatsu from Toyama University teaching about the setting and maintenance of a Japanese plane (kanna). I do not own, nor I claim, any rights on these videos - all copyrights are owned by the respective owners.
The straight edge (shita-ba-jougi) used to check the bottom of a dai (shita-ba) must be always perfectly straight.
The reason why these straight edges are in a set of two is because you always repair them together. We pair them together, and plane them together. Then we check them one against the other.
Here and here you can see some light, which means that we need a plane a little more.
There's still a little bit of light passing through here, but everywhere else is perfect. At this stage, we back the blade as much as possible to shave very little material, and we remove both sides.
Now it’s perfect, there is absolutely no light passing through.
This was made by a friend of mine. It is called okuriari. It slides like this and this is done to force it to remain perfectly flat.
Usually the ones you buy are like this, with two pins that hold them in place, but this way each of the straight edges can warp in time. This one, for example, has a bit of space in the middle.
This is a very important tool.