Back in the early 1980s, DOS filenames couldn’t be more than 8 characters long with a period and a 3-character extension. That was limiting, so when Apple developed the Mac operating system in 1984, it allowed longer names and eliminated the need for an extension, although Mac OS X’s Unix roots meant a return of the filename extension in 2001. Since then, filename restrictions have loosened to the point where it’s easy to think that they no longer exist.
If only that were true! In some ways, the situation has become even cloudier, thanks to additional limitations from file-sharing services like Dropbox, OneDrive, and Box. (Google Drive’s native Web interface reportedly has no naming limitations, but files whose names contain Windows or macOS forbidden characters may not sync via Google Drive’s desktop software.) Plus, people tend to move files between operating systems more than ever before—if you’re sending a file from your Mac to a Windows user through Dropbox, you need to make sure that all three can deal with the filename.