What causes "ring around the collar"? How does one prevent/get rid of it?

Ring around the collar is a combination of sweat and body soil that rubs off the neck and onto the inside collar of a shirt, leaving that telltale dark line. On a white dress shirt, pre-treating is key, and we have a great product that makes this easy and convenient—Clorox^® Bleach Pen Gel. It has the same active as Clorox^® Regular-Bleach, but at a lower level, so it’s safe to apply directly to bleachable white fabrics. Apply a little gel directly to the stain, gently rub it in with the soft scrubber tip, and then wash the shirt immediately in hot water using detergent + ¾ cup Clorox^® Regular-Bleach. You can prevent the stain from building up by treating it each time you wash the shirt, and it is also good to only wear a shirt once before washing.