Abstract: An alignment and spacer apparatus includes an elongated body member that extends longitudinally along and laterally from a longitudinal axis. The elongated body member has an arcuate surface and a flat surface facing opposite the arcuate surface. The arcuate surface curves about the longitudinal axis and has a longitudinally extending crown. Further, the body member includes at least one stop tab that projects from the arcuate surface to define a stop tab surface. The stop tab surface extends laterally and perpendicularly relative to the longitudinal axis and at least a portion of the stop tab surface protrudes above the crown. Also described is a siding panel installation system for installing siding panels on a support structure which includes a plurality of elongated body members. Each body member includes a sequence of stop tabs that are spaced equidistantly apart from one another and project from the arcuate surface.