There us good info at Marathon thread. There are differnt weight & uses. For piecing 50wt/3ply or 50wt/2ply works very well. Thicker thread takes up more room thus altering your 1/4" seam. For decorating/quilting depends on project. I uses lots of embroidery thread-rayon- metallic & poly when doing decorative stitches on candlemats/tabletopper. (I make lots, I do craft shows I have Brother, Viking Janome Singer & Elna & don't seem to have problems even with C & C or metallics.