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Sewing: One of the controls on a sewing machine. It adjusts the pull the machine exerts on the upper thread (the one that goes through the needle) while sewing.

The relationship between the bobbin thread tension and the upper thread tension, in combination with the thickness of the fabric being sewn, affects the appearance (and effectiveness) of the stitching. Since bobbin thread tension cannot be changed on the fly, and you're presumably using your fabric for a reason, the upper thread tension is the one to adjust.

If you suspect your upper thread tension is wrong, make a sample line of stitching through all the thicknesses you intend to sew. Look at the stitches you've produced.

  • If stitching on the top of the fabric looks normal (like a dashed line), but the underside has a bunch of little loops with the bobbin thread running through them, your top thread is too loose. Increase your thread tension. Note, however, that this is the ideal adjustment for gathering fabric.
  • If the top of the fabric shows little loops, decrease your thread tension.
  • If both sides show normal stitching, then you've got the right thread tension.

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