You are right, it is tough to tell the difference, but I am pretty sure this is sil. It has the frayed edges as Jeff describes earlier, holds water overnight, and I cannot blow thru it. I think I got lucky this time. It also is identical to the sil that I purchased from Thru-hiker.com a few months back and to the sil that my MacCat tarp is made of . . . now to figure out what I want to do with the stuff! Lots of possibilities but such limited sewing skills!!