If the upper thread tension is too tight or too loose, the resulting seam is not strong. The tension of the thread must
therefore be adjusted accordingly.
If the tension is correct, the threads cross in the middle of the textile.
If the tension is too tight, then the lower thread is visible on the upper surface of the fabric and the fabric can become gathered.
If the tension is too loose, the crossing of the threads is visible on the upper surface of the fabric.