Abstract: Embodiments of a stairway or walkway handrail construction tool are shown and described, each embodiment comprising a frame of four outer members. The members are pivotally joined at or near each end and can thus be moved to form a square or rectangle and variously-angled parallelograms. Whether used on sloping stairs or flat floors, the top and bottom members of the tool will be parallel to each other and to the corresponding stair slope or floor. Thus, the invention may be placed temporarily in the subject location, it may be adjusted up or down according to taste or other criteria to chose a particular configuration for the prospective rail, and then the inventive device may be used to mark, align, plan, and/or visualize the layout of the railing or other feature for its construction. The preferred tool may be fully collapsed to a nearly-flat configuration, to make it very portable and amenable to transport within a case such as a carpenter's level case.