The bobbin thread for embroidery should be a bobbin-fill thread.  This is a fine thread that is usually black or white and sits close to the underside of the work and should never appear on the top side at all. It should have enough tension to pull the coloured top-thread firmly to the underside, but it still should be visible when the back of the stitch-out is viewed. It is essential to gain the best results that you use the bobbinfill that is recommended by your machine manufacturer (this may also form part of the terms of your warranty). To test the tension on a machine and to make sure that the top thread and the bobbinfill works well together, use the following formula:

Firstly, with a block font and in uppercase, create the word ‘FOX’ on either the computer software or with a font built into the machine. The type of fill-stitch required for this test is a satin.  The characters need to be at least 3cm (1¼ in) tall to get the right proportion for the satin stitch to show on the underside of the work. Use a new needle (80/12 or 75/11 embroidery) in the machine and use a quality thread with the bobbin-fill in the bobbin.  There needs to be a medium-weight backing to support the washed calico (or similar) in the suggested hoop for the size of the word.

Most machines will either give a guide for the top tension setting on the machine or the machine itself will automatically default to its own embroidery tension setting. When the stitch-out is complete, remove the hoop and turn to the underside of the work. If the machine’s tension is correct the bobbin-fill thread should only make up a good third of the under-pattern and should not venture too close to the outer perimeter needle holes.   

The word ‘FOX’ gives every possible angle the machine may have to stitch while creating an embroidery design and this is a great exercise to perform if you’re not sure if your tension is correct. Tweaking (reducing or increasing) a machine tension is not necessary but can make a huge difference to a finished stitch-out.  However if your tensions require constant ‘tweaking’ then a service by an authorised technician is recommended.