In the US and I believe one or two others the masking pen is 91.
The tip Karl O suggested,which works a treat, is to have pen 91 and pen 19 in all pen sets set to white masking fill.
Erika
Architect, Consultant
MacBook Pro Retina, 15-inch Yosemite 2.8 GHz Intel Core i7 16 GB 1600 MHz DDR3
Mac OSX 10.11.1
AC5-18
Onuma System
"Implementing Successful Building Information Modeling"