Yarn quality: what to check, and how bad yarn shows up in cloth

Yarn quality is judged on six things. These are how consistent the count is (count CV%), how strong the yarn is (CSP for a skein, RKM and single yarn strength for individual threads), how even its mass is along its length (U% or CVm%), how many thin places, thick places and neps it has per 1,000 metres, how hairy it is, and whether the twist is right and steady. Each one has a fabric consequence. Count variation shows up as barre bands, unevenness as streaks and a cloudy look, weak spots as warp breaks and holes, neps as specks after dyeing, hairiness as pilling and fuzz. Most of these faults cannot be fixed once the yarn is woven, so they are cheapest to catch on the yarn test report.