Top Thread Showing on the Underside: Fix the Loops
Loops and long floats under the fabric mean the top thread lost its grip upstream. Here are the five checks that fix it, in the order that finds it fastest.
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Birdnesting: Why Thread Piles Up Under Your Fabric
That mat of tangled thread under your seam almost never starts in the bobbin. Trace the top thread, run five quick checks, and stop the nests for good.