Most of us use thread count -- that is, the number of threads per square inch of fabric -- as a proxy for quality. And it is definitely important -- just go to any good linen department and give the samples of different thread count a good stroke and scrunch. But it's not the only thing that matters, nor is it what matters most: the quality of the fiber matters more. (You could have a sheet of densely woven straw, for example, and you wouldn't want to sleep on it.) Egyptian cotton is most used in luxury linens because it produces a long fiber, or staple, which means it's easier to spin into smooth, fine yarn.