is it a single layer, is it a fitted twin sheet

The Cocoon Silk Travel Sheet is not your typical flat sheet that you would find in the linen section of a department store. It does lie flat but it has a double stitched seam on one side and an opening on the other with a velcro closure. It also has an insert at the top that acts as a pocket for your pillow. It packs down VERY small. Its full dimensions when laid out are 86" x 35" (220cm x 90cm) and it weighs just 6.3oz (180g). When you stay somewhere where you do not trust the sheets this is a great item to have with you. I know international backpackers that use a variety of hostels in 3rd world countries who refer to these as "Bed Condoms." While I don't know how effective they are in preventing the spread of disease, the Cocoon Silk Travel Sheet definitely offers some degree of comfort and peace of mind. While it can be used to add warmth to a sleeping bag (the tag clams it adds 9.5F or 5.3C). The Cocoon Silk MummyLiner that Josh is referring to below http://www.backcountry.com/store/COC0009/Cocoon-Silk-MummyLiner.html does not have a velcro side or pillow insert. It really is just a liner for a sleeping mummy bag. A true bag condom. ;-)