Underspun yarn has a low twist-per-inch ratio, making it weak, drafty, and prone to pilling or breaking under tension. While slight underspinning might be intentional for creating soft, lofty knitting yarns, excessive underspinning in warp yarns will cause them to snap during weaving. It occurs when the flyer or spindle speed is too slow relative to the drafting rate.
This batch of merino is completely underspun, it keeps drifting apart the second I put any tension on the warp beam.
Related terms
Plying
The process of twisting two or more single strands of yarn together to create a stronger, balanced thread.
WPI
Wraps per inch, a method for measuring the thickness of a yarn.
Yarn-beam
A large cylindrical spool that holds warp threads under tension for weaving.
Mule twist
A historic type of cotton yarn spun on a spinning mule.