Dunster & Prism both gave excellent information! The only thing I could add is that I do like Aurifil's nylon monofilament (invisible) thread, as far as invisible threads go. I have almost no experience with it, though, because mono is generally the most finicky thread on the market -- and nylon mono is even more finicky than poly mono (though, if you are willing to put in the time, I feel nylon gives a better result). If this is your first time working with mono, I would definitely recommend starting with a poly mono. Superior's MonoPoly is a pretty good mono if you're shopping online or at LQS; Sulky Premium Invisible Thread from JAF is something I've used in the past. To be honest, the Sulky mono did not hold up well to repeated washings over time. I made a table runner with it (SID) & after maybe 15-20 washes, the stitches started snapping. I re-quilted it with matching cotton threads.