Some machines have "self adjusting tension". For those of us who don't have these types of machines, we need to pay particular attention to thread tension, especially at the beginning of a new project. Imagine the needle goes down into the fabric and a sharp hook catches the needle thread and wraps the bobbin thread around it. If the needle tension is stronger than the bobbin tension the needle will pull the bobbin thread up through the top side of the fabric. Likewise if the bobbin tension is too strong, it will pull the thread through the fabric to the underside of the material. Perfect tension is where the threads locks together between the top and the bottom layers of fabric.
These videos will show you how to determine if your tension is set properly.
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