For attaching wood trim around a door or window frame. In the disclosed system, plastic spline-clips attached to the jambs serve to fix the jamb to the wall. The spline-clips have two splines, which engage slots in the wood trim, forming a dovetail connection between the trim and the splines. The splines are elastic enough to deflect apart to admit the trim, and then spring back to grip the trim and to push the trim tightly against the wall. Grooves cut in the edge faces of the jambs receive lugs on the clips, by which the clips are accurately positioned on the jambs. Also disclosed is a split-jamb system, in which the inside jamb-frame is positioned first, then squared up in the wall opening, and then held in place on the wall by means of with its inside spline-clips; the outside jamb-frame is then assembled to the inside jamb-frame, and the outside jamb-frame locked in place with its outside spline-clips. Also disclosed is a corner-cover, which masks the ends of converging baseboards and other trim, on both inside corners and outside corners. The corner-cover avoids the need for mitring the baseboards. The corner-cover is profiled to fit the profile of the baseboard. Also disclosed is a system for attaching trim, e.g baseboards, chair rails, to a wall. This includes a hook-clip, the hook of which engages a slot cut in the trim. The hook is springy, and presses the trim against the wall. The system biases the trim so that it is the top of the trim that is pressed against the wall.