With the entire cover being stitched together, the thread is also an important aspect to consider (some cover manufacturers heat seal their vinyl together, but the seams tend to become very brittle in the hot sun and freezing cold).
The thread used in our covers is a 93#, UV resistant, bonded, Dacron Polyester thread. The polyester part is what important, unlike cotton thread it won't rot out in a short period of time, and the UV resistance prevents any degradation form the sun. "Bonded" means that it's actually water tight and can't absorb mildew causing moisture. The 93# is the weight of the thread and the entire cover is double stitched (except for the hinge which is quadruple stitched).