Abstract: A tool for aligning vinyl corner posts during installation has a flat, generally L-shaped body member. The body member is formed from first and second leg portions which meet in a 90.degree. angle. An aperture is formed in the interior edge of the body member which is symmetrically disposed with respect to the 90.degree. angle for receiving a vinyl corner post. To use the tool, the vinyl corner post is pushed into the aperture where it fits snugly. The assembly is then pressed against the corner of the building, and the flat interior edges of the legs of the body member are pressed against the sides of the house to align the corner post to the building. The corner post is then nailed into position, while sliding the tool on the corner post to maintain the proper orientation along the entire length of the post.