— Patrick, Roseville, CA
A: Kevin O'Connor replies: According to a couple of carpet-cleaning companies we checked with, that sort of stain is fairly common on all wall-to-wall carpeting; it just shows up more on light-colored ones. The stain is typically caused by a gradual build-up of dirt, left behind because vacuum cleaner heads can't reach into corners. So the best way to prevent the staining is to use a crevice attachment along the baseboards every time you vacuum. Unfortunately, this build-up is difficult to remove once it becomes visible; a professional carpet cleaning may be your only option.
By the way, a similar-looking carpet smudge sometimes shows up at the bottom of interior doorways, baseboard heating registers, and baseboard trim that isn't flush with the floor. This is called filtration soiling because the carpet fibers filter out dirt and pollutants from the air flow. This kind of dirt is generally easier to remove than what you have.