Very fine, flexible polyester monofilament which is on an easy to use snap spool that will deliver the thread easily off both horizontal or vertical spool pins without the thread spilling off. No rough edges on the spool to tangle, snarl or break the thread. Does not melt with normal ironing through cotton setting. Soft, strong and smooth for all your invisible sewing needs. Works well in the bobbin too. Also ideal on Sergers.