You can draw very clear lines on fabric with these gel pens. The line disappears if you iron it. The colour if the ink disappears with heat, and will return if you were to put your fabric in the freezer (in cast you have ironed a pattern you still need). Always check the fabric beforehand, because a faint white line may remain visible on dark fabrics. I use these pens on light and medium fabrics, ideal for marking quilting and sewing lines.
For dark fabrics, the white marker pen by Clover is the right choice.
Refills for these pens are also available.