A Webb mask is a transparency that you overlay on the screen of an oscilloscope to be able to measure the phase between two signals. Newer digital scopes have this functionality built-in, but older analog scopes need a Webb mask.
This tool prints such a Webb mask customized to the size of your scope screen. It is written in PostScript, a language in which it is straight-forward to specify actual printed sizes. It was written as an extra credit assignment for EE 113: Feedback and Control Circuits.
To customize the mask for your oscilloscope, measure the size of your display. It is assumed the size is square. Then open the webbmask.ps file in a text editor and look for the following lines:
% screen size in inches
/screen 5 def
% display size in inches
/screen 4.5 def
Change these to match your oscilloscope. Then open the file with your favorite Postscript or PDF viewer and print it onto a transparency. Cut it out and lay it on your oscilloscope screen, using a bit of tape if necessary to keep it in place.