Qualities of Fabric - Drape

http://www.ravelry.com/patterns/library/the-mayfair-top

One of the hardest things to demonstrate to knitters through words is how much drape does a fabric have. Yet it is critical to the results of our knitting projects.  What we are trying to capture is how fluid or stiff the fabric is.  It's is not about the thickness of the fabric, it’s an assessment of the manner in which the fabric moves.  To begin to understand this concept think of the garments in your wardrobe. If you own an Oxford style shirt in a stiff cotton compare it to a silk blouse. Look at how the collar stands on it's own in cotton but falls against the body of the blouse when created out of silk. With knitted fabrics we see similar qualities dependent on fibre, stitch patterning, spinning method and gauge. In some ways I think this is more complicated for knitters because we have such varying results when we create our fabric. When you buy clothing you put it on and the assessment is pretty quick. On the other hand sometimes the fabric qualities change after washing or dry cleaning. This is usually due to fabric finishes. Examples would include Teflon to cut down on staining, or low wrinkle treatments on cotton shirts. 

Take a look at my photo at the top, one sweater is knit in cotton and the other is in rayon. You should be able to tell in this photo which is which. 

Hint, look at how the sleeves hang and at the hems.

I hope this helps your understanding.