Origin of Denim
The most common denim is indigo denim, in which the warp thread is dyed, while the weft thread is left white. As a result of the warp-faced twill weaving, one side of the textile is dominated by the blue warp threads and the other side is dominated by the white weft threads. This causes blue jeans to be white on the inside. The indigo dyeing process, in which the core of the warp threads remains white, creates denim's signature fading characteristics.
Denim was traditionally colored blue with indigo dye to make blue jeans, although "jean" formerly denoted a different, lighter, cotton fabric. The contemporary use of the word "jeans" comes from the French word for Genoa, Italy (Gênes), where the first denim trousers were made.
Jeans can be classified by their weights -12 Oz. or less is considered light denim, 12 Oz. to 16 Oz. is considered mid-weight, and over 16 Oz. is considered heavy weight. Heavier denim is much more rigid and resistant to wear, but can also take more wears to break in and feel comfortable.
Below is a chart which shows us the further classification of jeans : -