Somebody I know expressed irritation today regarding those of us who tend not to use spaces in our filenames, e.g. NamingThingsLikeThis.txt -- despite most modern operating systems supporting spaces in filenames. Non-technical people must look at filenames created by geeks and wonder where we learned English.

So, what are the reasons that spaces in filenames are avoided or discouraged?

The most obvious reason I could think of, and why I typically avoid it, are the extra quotes required on the command line when dealing with such files.

Are there any other significant reasons, other than the practice being a vestigial preference?

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UPDATE: Thanks for all your answers! I'm surprised how popular this was. So, here's a summary:

Six Reasons Why Geeks Prefer Filenames Without Spaces In Them

1. It's irritating to put quotes around them when referenced on the command line (or elsewhere.)
2. Some older operating systems didn't used to support them and us old dogs are used to that.
3. Some tools still don't support spaces in filenames at all or very well. (But they should.)
4. It's irritating to escape spaces when used where spaces must be escaped, such as URLs.
5. Certain unenlightened services (e.g. file hosting, webmail) remove or replace spaces anyway!
6. Names without spaces can be shorter, which is sometimes desirable as paths are limited.