A mountable welt that is easier to attach to a desired surface or joint. The welt includes a transparent or semi-transparent sheath casing circular or oval in cross-section with at least one integrally formed, laterally extending wing member. The wing member is a relatively thin structure that extends longitudinally over a portion or the entire length of the outer casing. In the preferred embodiment, the outer surfaces of the wing member converge towards their outer edges thereby enabling the wing to be placed into a joint space. A suitable attachment method, such as sewing, an adhesive, or a closed joint, may be used to attach the wing to the desired surface or joint while allowing the outer casing to be exposed. Located inside the outer sheath is a continuous linear light.