For use with horizontal siding panels each having complementary first and second locking means respectively formed along their top and bottom margins for interlocking vertically adjacent courses of panels in overlapping array, a system for mounting, on a wall, a panel interposed between already-mounted upper and lower courses which are vertically spaced by a distance less than the height of the interposed panel. The system includes clip means for securing the second locking means of the upper-course panels to the wall while permitting upward insertion of the first locking means of the interposed panel, behind the upper-course panels, to a level above that at which the second locking means of the upper-course panels would interlock therewith, retaining means for securing the first locking means of the interposed panel to the wall at that level against upward, downward or outward movement, and means mountable on the top margins of the lower-course panels for interlocking with the second locking means of the interposed panels, the interlocking means, when thus mounted, being vertically slidable through a range of elevations relative to the lower-course panels.