The .msk files can be used to control transparency of a .bmp. It's covered in VFP9 help under How to: Enhance Control Display.

>I always see corresponding .msk files for most .bmp files in the VFP folders. How are the .msk files used?
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>I usually just edit the .bmp files in Paint and set the Opaque property off then fill the background with a White color and save the same .bmp file. This makes the .bmp transparent in the VFP controls.