Abstract: A tool for marking a cut line on a panel, which panel is to be fitted into a gap, on a surface, between a pre-fixed panel and an adjacent edge of the surface. The tool comprises a manually-grippable base that defines a marking datum, first and second arms that are mounted on the base for linear sliding movement parallel to each other, and locking means to secure each arm to project from said datum by a preset distance. In use a panel to be marked may be laid between the pre-fixed panel and the adjacent edge, the tool's arms being slid to and locked at a required setting. The tool is then slid along said adjacent edge with the free ends of the arms in contact therewith while the marking datum is used to mark a cut line on the panel to be cut.