This white marker is handy for 3 basic purposes:
A) Marking flat glass film prior to installation of windows with unusual shapes (arches, trapezoids, etc.) which must be cut to approximate shape before the final lay-up.
B) Circling a contaminant (hair or fiber) before pulling back the film to remove it.
C) Marking film edges to check for glass movement on side roll-ups during the trimming process.
After installation the white grease mark wipes from the surface of the film easily.