Ello!

I have a thick curve (10 pt). I need to remove middle part (8pt) of it (or make fully transparent) but borders (1pt on each side) must be seen.

Or maybe it's better to create one 1 pt curve and to offset it to each side? If this is better then how to do it?

Thanks in advance!

 Arrange - Convert Outline to Object

CTRL SHIFT Q

Now your outline is an object with a fill and a new outline you can adjust independantly.

All the ways you mention would work, I would do it this way:

If it's purley visual, copy and paste the 10pt line on top of it's self. Set the copy to 9pt with a white fill.

If you want to actually see something in the gap, convert both outlines to curves (arrange menu) and combine them.

Or just start with a 9.5pt line, convert it to a curve and give this curve a 1pt outline.

Rikk Flohr,  thanks!

I tried it your way and it's quite good :)