Even though I sew, doesn't mean that I stopped buying ready-to-wear garments, but I do feel like I pay more attention to the problems of garment construction.  I, one time, owned a pair of jeans and, everytime I wore it, I always noticed that one particular pant leg would always twist so that the inner seam would always end up on the front of my leg.  How annoying is that and why does it happen?  Well, the answer has to do with the grainline.  If you've done a commercial pattern before, you must have noticed the grainline arrow printed on the pattern.  In fact, the pattern layout directions show you the best way to layout patterns so that those grainline arrows are always parallel to the lengthwise grainline of the fabric, or on grain.  Of course, following this rule means that you'll be using more fabric, so companies may break the rule just to save money on fabric and cut pieces off grain.  The consequences of this is that fabric pieces are no longer sharing the same grainline and different directions of grainline have different degrees of stretchiness.  The result being that the fabric pieces may not necessarily hang with consistency and you may end up with pants like mine.

This tutorial goes over grainline like how to find it, how to make sure yours is straight and how to square yours.  By following the grainline rule, you'll be sure to end up with clothes that hang straight and wear evenly.