Handy for three basic purposes: 1) marking film prior to installation of windows with unusual shapes (arches, trapezoids, etc.) which must be cut to approximate shape before the final “lay-up;” 2) circling a contaminate (hair or fiber) before pulling back the film to remove it; 3) marking film edges to check for glass movement on side roll-ups during the trimming process. Yellow grease mark easily wipes away.