Please bear with me. Some knowledge about textiles will help you to more fully appreciate what vintage jeans are all about.
Denim is a fabric traditionally woven with what is known as a twill weave. This is simply a weaving pattern that produces parallel diagonal ribs. It is made by passing the weft threads over one warp thread and then under two or more warp threads. Weft is the yarn which is shuttled back and forth across the warp to create a woven fabric. In the USA, it is sometimes referred to as the “fill” or the “filling yarn”. The most common twill used for jeans is a 3×1. A 2×1 twill is used in lighter weight denim.
The yarns used in making denim have a very high twist, a process which gives the yarn much greater resistence both to tensile stress and to abrasion. The original dye used to color the warp comes from several species of plants, but nearly all indigo produced today is synthetic.
Traditionally, the denim was woven with a pre dyed warp and a natural (white) filling. The weft dominates the front of the fabric (3 to 1) and thus gives the appearance of an almost completely blue surface. The back of the fabric is dominated by the natural colored fill.
Some contemporary jeans are dyed in the fabric stage, thus producing a darker dye jeans that retain their color longer, as yarns in both directions are dyed. To produce this darker dye jean effect, sometimes the finished garments is dyed.
The twist of the yarn is so tight that the indigo dye can only color the surface, leaving the center fibers white. The enclosed white fibers are eventually exposed through wear and washing, as the superficial indigo is worn off. Obviously, the fabric will fade more in areas that are exposed to greater friction. It is this peculiar quality of denim, aside from its durability and comfort, that has so endeared it to consumers. It is this quality that has sparked the huge trend that vintage jeans have become.