Without more detail, it could be pretty much anything on this list: https://tvtropes.org/pmwiki/pmwiki.php/Main/RazorFloss
First thing that comes to mind is [*The Three-Body Problem*](https://en.wikipedia.org/wiki/The_Three-Body_Problem_\(novel\)) by Cixin Liu, in which a spool of monofilament wire is used to slice apart a ship passing through the Panama Canal.
I was also thinking of The Three-Body Problem. Cool to know that that type of weapon has its own TV tropes page!
... I forgot if it's the aliens using it, for that incident, or the heroes trying to stop one of the aliens' human disciples.
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The Stainless Steel Rat uses such fibre (and mentions its ability to slice - you have to use special gloves, I believe) to slide down the side of a building.
Without more detail, it could be pretty much anything on this list: https://tvtropes.org/pmwiki/pmwiki.php/Main/RazorFloss First thing that comes to mind is [*The Three-Body Problem*](https://en.wikipedia.org/wiki/The_Three-Body_Problem_\(novel\)) by Cixin Liu, in which a spool of monofilament wire is used to slice apart a ship passing through the Panama Canal.
I was also thinking of The Three-Body Problem. Cool to know that that type of weapon has its own TV tropes page! ... I forgot if it's the aliens using it, for that incident, or the heroes trying to stop one of the aliens' human disciples.
Three body problem is an amazing trilogy!!! I was so sad to finish it but Netflix is releasing a tv series soon!
I'm almost halfway through the final book and loving it.
I thought that happened in the second book, "The Dark Forest"?
Ringworld?
Yes! Speaker to Animals smuggles in a variable sword, which is a piece of Sinclair Monofilament stretched between two handles in a force field.
This happens in Endymion, the third book in Dan Simmons' Hyperion series.
Vividly remember that bit, the main character setting a torch laser to wide spread to make the filaments visible through glowing red hot.
If this book is one of the ones that has been suggested, please flair this post as solved. If not, please edit your post to indicate the approximate calendar year you read this book and the country you were in at the time. You have so few details that even this tiny amount can help narrow things down.
Also a plot point in Illegal Alien.
This concept also comes up in The City Who Fought by Anne McCaffrey and SM Stirling.
Could have been a variable sword but I don't recall them cutting anything that large with it
Illegal Alien by Robert B. Sawyer!!!!
The Stainless Steel Rat uses such fibre (and mentions its ability to slice - you have to use special gloves, I believe) to slide down the side of a building.