It's basically a sleeping bag, but a more high-tech version of one. It's made up of two comforters, one a heavier winter weight and the other a lighter summer weight. In cooler weather you put the summer comforter on the bottom and the winter one on top and when the weather warms up you flip the sack around. These two comforters have zippers on three sides so you can zip them up to create a sack or zip them completely apart for washing or zip them anywhere in-between. On the inside of each comforter, along each side, are continuous strips of velcro which are used to attach the one long flat sheet of the system to the insides of each comforter. This velcro holds the sheet in place while you're sleeping in the sack, but makes it very easy to remove the sheet for washing. I bought the twin size (42" x 80") and the blue matches my quilt perfectly. At $125, it's a pricey sleeping bag, but I like the entire system. I washed all the components yesterday and they all washed and dried nicely (the instructions say to line dry the comforters and just fluff them on Air for 10 minutes, but I actually dried mine on Delicate for 45 minutes and they came out fine). I had plenty of room to sleep in the sack with the sides zipped up and when I got too warm overnight it was easy to unzip one side and throw the cover back.
Now what I've done is I have the bed made up with just the quilt over the mattress and topper and use the bed as a sofa for seating during the day. At night I'll just unroll the Travasak and slip right in to sleep. In the morning I let the Travasak air out a bit and then roll it up and use it as a bolster on the sofa. A nice, neat home and no more broken back.