To the best of my knowledge, since I don't actually own a set, they are entirely a set of interchangable circulars. They come with different things you can put on the ends to use them like flexible single needles (I think). I don't think that's a bad thing though since you can use circular needles to knit the same way you would with straight needles.
Here's the link to their site. You can read about the history there if you are interested.http://www.knitdenise.com/