In our sewing group we often use an # 8wt. Perle cotton, sometimes a #5 wt. I like the Perle cotton for tying them, it seems like you get tighter knot that lasts. We always tie a double square knot.

The trick to tying a knot that won't come undone is to do a square knot. Here's a link to a video how to do it:

https://boyslife.org/outdoors/outdoo...a-square-knot/

I have used embroidery cotton and even a good strong yarn. Yarn works very well and holds well. I space about every 4". Tying on a Crazy Quilt In my opinion is the only way to go. I haven't tied a quilt in more than 10 years with the last Crazy Quilt I made.

I only tie comforters and use crochet thread, doubled and a large needle..

Oh...I forgot...I use one of those old fashion dental floss loopy things....to thread my larger eye needle when I tie quilts....there the ones that just have a plastic loop with a tail...some people used then to thread the loopers on their sergers too...

There are some novel ways to tie a quilt. There is a You Tube video of the hidden tie stitch. Jen Kingwell has a demonstration of a variation of hidden stitch that is an X or a cross. I don’t have a link, but a google of Jen Kingwell should find it. Both of these methods avoid the strings.

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.🌞

I like to use a doubled crochet cotton and tie with the "international stitch." If you want to know more about how to do this stitch, check out this youtube presentation.
https://www.youtube.com/watch?v=cEkHht2oJsg

Thanks for the link....that looks like a good method also to do....especially on a baby quilt....it might be a bit more work on a full size quilt as you get to the middle of it as you have to reach under the quilt …..I have also tied a quilt with the machine using a tacking stitch or a decorative embroidery stitch. My D1 and SE had lots of nice stitches for that....

now that International stitch is great. the surgeons or double knot didn't work for me with embroidery thread. But i may try this one soon, just dont' have that long of a needle!