http://en.wikipedia.org/wiki/Image:Guitar_neck_back.jpg
"Some electric guitars (like the Fender Telecaster and Stratocaster) have a dark wood inlay contrasting to the light colored maple running on the back of the neck, commonly referred to as a "skunk stripe". Strictly speaking it serves a structural function, unlike merely decorative inlay, because it covers a cavity for the truss rod that runs the length of the neck. With one-piece maple necks the truss rod is fitted from the rear of the neck, as opposed to being fitted from the front where it is covered by a separate fingerboard. Even though some makers fit the truss rod from the front of the neck and do not require a "skunk stripe" on the rear, they emulate this appearance by inlaying a purely cosmetic "skunk stripe"."