Abstract: A mat opening marking device is provided for laying out full line rectangular openings to be cut in picture frame mat material, and the like. A thin, transparent plate member on one side mounts a straight edge member which can be adjusted to various parallel positions to engage an outer edge of the mat material and on an opposite side mounts a marking device support member having a replaceable marking device whose marking point can be arranged to extend through any one of a series of arcuately arranged selected openings in the plate member to engage and mark the four sides of the mat surface opening along the four outer edges of the mat and at predetermined distances inward therefrom.