Abstract: Apparatus including jig means and associated scribing means for scribing a wall panel for insertion into a wall area bounded on one side thereof by an already mounted wall panel and on the other side by a wall corner. The jig means is preferably of overall S-shaped configuration and includes a pair of oppositely disposed open ended slots, one of which receives therein the proximal edge of the already mounted wall panel. The base portions or inner ends of the slots overlap so as to be disposed a predetermined distance apart from each other, such that the roughly sized wall panel to be scribed may be disposed in the other slot so that the panel to be mounted overlaps the already mounted wall panel a distance equal to said predetermined distance. The scribing means includes a reference line follower and a scriber spaced apart from each other said predetermined distance such that a trim line may be scribed said predetermined distance inwardly from the other end of the panel to be mounted.