Help! Need Help With A New Term

I am looking for help with a term that I am not familiar with at all. I received stamp booklets (Scott BK 157 as an example) that show in their description the line "ohne Nutlinie". Others say "mit Nutlinie", possibly a German term. I have been searching the internet, but no success. Now I am hoping that somebody can help me with this term.
Thank you very much in advance.
Pete

Google translate says it's "without groove line" in German. Not quite sure what that means, unless it refers to a guideline.

Further......
Could "groove lines" mean "creasing" perhaps ?

Scott No. 2CVP15-22 Var. PictureItPostage Stamp Set No. 1
of all 4 value levels 23 ¢ to 60 ¢ horizontal and vertical
without groove lines on the back of the carrier film.
http://www.usstamps.de/cvp/picturei...20051101.htm

Creasing is a real possibility, since there are booklet panes that are known both folded (creased) and unfolded.

To the best of knowledge it means folding line.

Part of the paper(in this case booklet) that can easily be bent.

In this case to be able to also close the booklet.

Yes, it is a "line" where the material is a bit thinner (or pressed) so that it can be folded.

Thank you all very much for the interesting replies.
In my opinion the replies are all very much alike, which made it easy to look for the "nut line". What I have noticed is that all booklets WITHOUT the nut line include the number of stamps on one pane that make up the length of the cover. Therefore, a booklet WITHOUT the nut line is one where the pane is not folded over. On booklets WITH the nut line, I found that they all contained one or more panes that were longer than the length of the cover, hence had to be folded over. It seems than that the nut line is the line where the pane is folded over. If there are still other explanations I'm interested in hearing about it.
Again, thanks very much.
Pete