I wrote two or more unicursal straight lines.
Next I did "Path-Combine" them, and I tryed to join two endnodes,
which were in another unicursal straight lines.
In some cases, I could join, but in some cases, I failed.

I guess only if joined lines will be unicursal, I can join.
Please help me.

Well, I have to admit, I had to look up the definition of "unicursal". And I don't clearly understand it. If it's what I think I understand, Inkscape is not capable of creating non-unicursal lines. So there should be some other reason for some of the Joins not to be working for you. However, please note that I''m not sure I understand unicursal.

That said though, I should still be able to figure out the problem. First, what version of Inkscape are you using?

In this case, there are several different potential causes for this problem, and it might be easier if you could just upload your SVG file, so that one (or more) of us could open it, and look for the problem. But first, let us know your version

Hi brynn, thank you very much.

I am using Ver 0.47. I do not use Ver 0.48 because I had a problem with textext.
I am sorry for not explaining "unicursal". But it is difficult to explain by language
(I am Japanese, so my English is very poor. Now 8:25AM).
My dictionary explains "unicursal" like the following.

relating to or denoting a curve or surface which is
closed and can be drawn or swept out in a single movement.

If InkScape do not support "unicursal lines", I guess
it is due to endnodes structure(that has only one next endnode information.)

How can I upload SVG files?

I am so sorry for annoying question.

No worries, question is not annoying

Ahhh, well since you're using 0.47, you may not have to upload.
Please select one of the nodes that will not be joined to another, and look in the info/status area (bottom of window, in the middle) (http://tavmjong.free.fr/INKSCAPE/MANUAL ... atomy.html). The type of node selected will be described there. If it's an end node, it will say "end node" plus some other stuff.

(For some reason, version 0.48 was released without some of this very helpful info -- node description info. If you were using 0.48, it would be easier to examine your file, than describe troubleshooting steps. But with 0.47, we should be able to fix it without upload. Although, we will see what happens )

Be sure to look for all of the nodes that will not be joined together. If they ARE all end nodes, please try to join them again, and watch the info area when you click the Join button. It will probably give you some helpful information about what's happening or not happening. And then please let us know what it says.

If they are not end nodes, you can probably figure out what to do. But if not, please don't hesitate to reply again, and we'll keep trying

Fyi, this is the definition I found for "unicursal", from The Free Dictionary

That can be passed over in a single course; - said of a curve when the coördinates of the point on the curve can be expressed as rational algebraic functions of a single parameter

I understand "single course". But "rational algebraic functions of a single parameter" is harder to understand. I DO understand it, but I don't understand why such a curve must follow a single course. Fortunately, you are talking about a straight line, so I don't have to try too hard to understand, lol

Hi brynn

I am so sorry, I tried to join non-end nodes(Henceforth I will call then mid nodes).
Inkscape treats all objects as bezier curves, so I guess we can not join
mid nodes.

So I have another question. If I want to move some mid node to another mid node
precisely(I want to overlap them), how can I achieve that?