I use either embroidery floss or yarn. A square knot is a must to keeping the knots tied. My Granddaughter who is 15, just completed a quilt and the teacher suggested tying their first quilt and she used embroidery floss. I'm just glad they are bringing back a few practical classes in school Home Economics. Of course, they don't call it Home Economics anymore. Her class was called Textiles and Quilting. She was the only one in her class that knew how to operate a sewing machine. I gave myself a pat on the back for that.🌞