What does the thread tension on a sewing machine primarily control?

The thread tension on a sewing machine plays a critical role in determining how the threads interact with each other when stitching. Correct thread tension ensures that the thread from the needle and the thread from the bobbin come together properly to form a balanced stitch. This balance is crucial for achieving neat and consistent stitches that hold the fabric layers together securely.

When the tension is set correctly, the upper thread and the lower thread will interlock smoothly at the fabric's seam, producing a straight stitch that is neither too loose nor too tight. If the tension is incorrect, it can lead to issues such as puckering, skipped stitches, or thread breakage, impacting the overall quality of the sewing project. Thus, controlling thread tension is essential for successful sewing outcomes, making it a fundamental aspect of operating a sewing machine effectively.