Denim is made from rugged tightly woven twill in which the weft passes under two or more warp threads. Lengthwise, the yard is dyed with indigo or blue dye; horizontal yarns remain white.
The yarns have a very solid turn to make them more hard-wearing; however, this also distresses the denim’s color.
The yarns are twisted so intensely that navy blue dye usually colors only the surface, leaving the center of the yarn white.
The blue strands turn out to be the threads that show on the outside of your denim and the white are the ones that make the inside of your denim that look white. This produces the familiar diagonal ribbing identifiable on the reverse of the fabric.