The opening created in a loom when the harness frames reverse their positions from the previous pick. If the first shed raised the odd warp threads and lowered the evens, the countershed raises the evens and lowers the odds, locking the newly inserted weft yarn into the fabric structure.
Heard on the job
The loom is jamming because the harnesses aren't dropping fully to form a clean countershed.
Insider tip
A clear, wide countershed is essential to prevent the shuttle from catching and breaking warp threads during high-speed weaving.