SINGER W8910 - Manual (Page 15)

Thread tension
Correct and incorrect thread tension
In order to easily imderstand the importance of correct
thread tension, you can try different thread tensions by
sewing on a scrap of fabric. Begin by using an excessively
loose tension, i.e. set the tension dial near “0”. Look at
the fabric and you will find that the lower thread lies
straight and the upper thread is drawn underneath the
fabric. If, however, you set the dial at the highest
number, the opposite will occur, or the seam may
pucker.
The thread tension is correct when the threads inter­
lock in the middle of both layers of fabric.
Please note that even the best quality thread may vary
in thickness. When sewing in fine fabrics, the knot may
sometimes be'visible.
Always check that the thread tension is correct by first
sewing on a scrap of the same fabric, folded double, that
you intend to work with.
If you set the stitch length to less than 1 mm yourself
for sewing close zig-zag stitches, you should also
decrease the thread tension by setting the dial at 4.