There are some factors to what makes a great denim fabric and one of these factors are besides what cotton type, also how the cotton or rather cotton yarn is spun. When speaking of yarns used for denim there are generally three ways. The first being Ring-spun denim, second Ring-Ring denim or double ring-spun denim and finally Open-end denim.
Ring-spun denim was the most common method used up until the 1970s. In this method the cotton yarns are twisted together for a stronger and harder-wearing product, which will also fade like the actual vintage jeans.
The Ring-Ring denim creates the best quality denim, as it employs Ring-spun yarn for both the warp and the weft. This is the traditional way of producing denim.
Open End was introduced in the 1970s and it reduced the cost of producing denim, as well as reducing the quality in many ways. The cotton fibres are mock twisted by blowing them together. This creates a fabric that is more coarse, dark (because it absorbs more dye), bulkier and it doesn't wear or fade as well as Ring-spun denim because of the extra dye.
It is possible to create a denim using a Ring-spun warp and an Open-end weft. This gives the fabric some of the strength and the look of Ring-spun denim, but it is still cheaper. It should be said, that just because you are paying large amounts for a pair of jeans doesn't mean you are getting a quality product. Some of the biggest designer brands are known for using Open End denim...