Recognizing balanced tensions
When the tensions are balanced, the stitched line looks good on both sides of the fabric, as shown in the top drawing at right, and the seam is at its strongest and most elastic. The easiest way to spot unbalanced tension is to look for visible knots or loops at the end of each stitch. When the bobbin thread shows on the right side, the needle tension is too tight or the bobbin thread, too loose, as shown in the middle drawing at right. When the needle thread shows on the wrong side, the needle tension is too loose or the bobbin thread, too tight, as shown in the bottom drawing at right. Of course, if you’re sewing on very thin or lightweight fabrics, both threads may show on both sides when the tension is balanced, simply because the fabric is so thin.

When upper and lower thread tensions are balanced, knot between top and bottom threads is hidden between fabric layers (top). When lower tension is too loose (or upper tension is too tight), knot is visible on right side (left). When upper tension is too loose (or lower tension is too tight), knot is visible on wrong side (right).

Tensions can still need adjustment even if they’re balanced. If both tensions are too tight, the seam may pucker, or break easily when stretched (test this on the more stretchy crossgrain, with at least a 6-in. seam). If both are too loose, the seam will gap when pressed open, exposing the threads between the sections.