I seem to see a great deal of the underscore in file names in design school - I thought the underscore was used before computers allowed you to just type normal file names (with a space between words). Is the underscore a MAC user thing?

So there is no solution of this problem.

I still do it out of habit

I think it is simply incorrect to use spaces.  Especially when you are dealing with a command line where a space traditionally signifies individual parts of a command.  A space in a file name becomes confusing in a command and you need to start using a delimiter (such as "") to mark the beginning and end of each file name in the command line which gets tedious and annoying.
 
You can use spaces if you want, but you just shouldn't.  Tricks were developed to allow you to use spaces in file names but that doesn't mean that you should use spaces.  It is especially incorrect to use spaces when you are dealing with web applications.
 
Simply put, if you use spaces in file names, the chances of an error occurring at some time as a result is increased.  You should get in the habit of not using spaces in file names.  For instance, if you are a graphic designer and your work goes on with spaces in the file name to someone who organizes your work to be used in a program or on the web, they will become increasingly annoyed with you.

bcavnaugh
Because _ is better then seeing %20 like in Test%20Me%20OK Test_Me_OK is easer to read. This is also web based files.

Yep, spaces in URLs are re-parsed to "%20" which is ugly.

Also (sometimes) when you put a link in an email body with space in the file name it fails...

Good example!

That is because the code running the web application is trying to automatically parse URLs and it thinks the space is the end of the URL and the remainder is part of the next word in the sentence.
 
Example:
http://stackoverflow.com/...s/15475645/20-is-added instead-of-spaces

Same here. My email address also still has the underscore......
 

boballee

Aside from the above, I still use the underscore to this day - even when saving documents locally.  Force of habit.

I use both space and underscore too, depending on the nature and intention of the file name.

I used the underscore between my words because in reality it makes the file name more as they say (Legal)
It helps ensure when working across platforms particularity online there is no issues with the spaces.
Also when working in a DOS or Command level on some systems like Linux when the filenames have spaces it very difficult working with the names.  Let's put it this way Microsoft Windows is very forgiving However if you have issues with your disk a file that is names without the spaces is a bit more easier to recover that one that has spaces.
 
Working for a major online university files cannot be uploaded with special characters like an apostrophe.
 
For me it just keeps it simple.  I LOVE simple.

Because when you write alot of batch files you get tired of remembering " "

As a programmer, it's needed sometimes :).
 
For example in Python you use __init__.py or in Helm you use _helpers.tpl to indicate a file that is not a Kubernetes spec, plus many many more examples...

i still use them instead of spaces. just habit i guess.