Abstract: A device of novel design employed as a support and guide during the installation of horizontally-disposed building siding, said device comprising principally a shelf-like component and a means incorporating a camming action for clamping the shelf-like component onto an installed length of siding. When two or more of the devices are clamped vertically at spaced intervals onto the uppermost length of installed siding, the shelf-like components provide a horizontal base on which the siding to be installed at the next higher level can be supported during installation, thereby enabling an installer to work alone. The shelf-like components further function to correctly position the siding to be installed relative to the siding already in place without the need for constant measuring and leveling. The device operates in a quick, easy and trouble-free manner, and it assures efficient, accurate and uniform siding installation.