Abstract: A tool that assists in installing lap siding. The tool includes a body having a handle and a pair of vertically spaced-apart seat assemblies that define a seat distance therebetween. The lower seat assembly is adapted to engage the lower portion of a previously installed siding piece, and the upper seat assembly is adapted to support the lower portion of a siding piece to be installed at the determined reveal distance relative to the previously installed siding piece. The reveal distance is at least substantially defined by the seat distance. In some embodiments, the tool enables two or more installers to with one hand properly hold and provide for level alignment of the piece of lap siding to be installed, while at the same time enabling the installers to mechanically fasten the same with the other hand. In some embodiments, the tool is adjustable to accommodate a range of reveal sizes.