Can songs made on the Windows version be opened on the Mac version?
Yes. SGU and MGU song project files made in a computer using one of the operating systems can be opened and modified in a computer using the other operating system.
A few users have indicated in other posts that they collaborate with users with computers running the opposite operating system.
As far as I know, all BiaB .sty files made on the PC can be opened on the Mac. I know for sure the MIDI styles made on the PC can be opened on the Mac because I've been selling aftermarket .sty and .sgu files made on the PC to both platforms since the early 1990s.
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