Because a zigzag is not a symmetrical stitch, and it is not meant to look like a straight stitch on both faces.
The geometry. In a zigzag the needle swings sideways between stitches, so the top thread travels further and is pulled across the surface at an angle. The thread also has to accommodate that sideways movement, which takes up slack a straight stitch never needs. The result is that the top thread naturally shows a little at the points of the zigzag on the underside.
What this means for judging tension. Set and check your balance on a straight stitch. If a zigzag then looks slightly different, that is normal. If the fabric tunnels or the points pull inward, that is a real fault.
When a zigzag genuinely needs a change. For satin stitch and dense decorative work, loosen the top tension slightly so the lock is drawn to the underside and the bobbin thread does not appear along the edges of the band. This is deliberate imbalance, and it is what makes decorative stitching look clean.
Fabric tunnelling is not a tension fault. Dense stitching pulls fabric in. The answer is a stabiliser underneath, tear away for wovens and cut away for knits, not a dial adjustment.
And fit the right foot. A satin stitch foot has a channel underneath so the raised band can pass. A flat soled foot rides on the stitching and drags it, which produces uneven results that look like tension.
Questions people actually ask
My zigzag pulls the fabric into a ridge.
That is the fabric, not the thread. Add a stabiliser and, on lightweight cloth, reduce the stitch width slightly.
Should I keep a separate zigzag setting?
Only for dense decorative work. For ordinary zigzag on seams and edges the straight stitch setting is normally fine.
Last reviewed 2026-08-11