What, if any, is the difference between an ESP and an esp ? (capital letters versus non-capital)

Is there any difference in how they operate? And why do some mods gets ESP instead of esp?
I seem to recall that the capital letters can cause problems, or are they actually identical?

Their identical to the game, but Wrye Mash can get confused between the two and not redate an esp if its extension was changed from capital letters to non-capital or visa-versa.

Thanks, Jac.

It's the first time I've ever created an "ESP"... but now that I look at my Load Order in Mash, I see that a few of the mods I trust are like that, and I've never had problems with them.

I think the best thing would be to test it: create a test esp with both .esp and .ESP to see how Wrye Mash handles it. I'm fairly sure that I've had redating issues because an esp went from .esp to .ESP and Mash said the plugin was no longer available.

AFAIK they're exactly the same. It does that to me too. So I just re-write .ESP as .esp when making a mod.

.ESP comes from Windows 7 (and maybe Vista; I don't know about that since I skipped Vista) liking to capitalize the file extension for some reason. Windows XP still has it in lowercase. It works fine, but as mentioned earlier, some programs that utilize case-sensitive processes might have issues.

But how do you re-write an ESP to be esp?
Windows doesn't let me do that. (it just lets me rename the file)

edit: yes, this is happening on my Vista laptop.
But my question still remains.

(Assuming you already have file extensions set to show)
You'll need to change the extension to something other than 'esp', such as 'es' or 'asdf', and then hit enter, and then change it again to 'esp'. Since 'esp' is the same lettering as 'ESP', Windows thinks you're not actually making a change.

Not sure, maybe the automated renaming in some Windows version perhaps depends on file name.extension length being <= old DOS 8 characters limit, so a shorter all upper case name may be used by the file system instead of a double (1 old DOS compatible + 1 longer Windows version) name. I remember it was something happening in some of the first Windows versions. Boring details http://en.wikipedia.org/wiki/8.3_filename

Windows/DOS file system is not case sensitive, so .esp is considered the same as .ESP or .Esp. If you want to upper/lower case a file/directory name, you have to change it to something else first, then change it.
example:
modname.ESP -> modname.esp2 ->modname.esp

Windows file type is determined by file extension (.exe, .txt, .esp ....), but file extension is not visible by default. Better http://www.mediacollege.com/microsoft/windows/extension-change.html to realize how things work and to be able to change extension part.

If your file is gonna be uploaded on some Linux based case-sensitive web host (very likely), better be able to see and change extensions to always use lower case IMHO.

These are two different extensions. Some programs recognize it as one, some not. If youre asking such a question, it means youre working in an environment which assumes they are one and the same thing. But it isnt always the case, especially today where you see a widespread use of multiOS cabable programs, for linux etc.

Windows is case sensiive too, its just disabled, enable it http://www.windowsitpro.com/article/tips/jsi-tip-7548-how-do-i-enable-case-sensitive-behavior-with-windows-xp-when-using-interix-2-2-or-windows-services-for-unix-3-0-.aspx.

Hmm... I would not define Windows as case-sensitive, I think on the contrary DOS/Windows are historically and philosophically case-insensitive. That hack could be of use in limited cases like that one described (need to write both upper/lowercase file at the same time), but I'd never use it as general setting, the risk to have problems with someother program relying on Windows being case-insensitive is too big.
[EDIT]typos

Its a simple reg setting. Like said, Windows is well aware of cases, but treats them as one. Whether it is a good thing or a bad one i cant say, nor particularly care. It certainly creates confusion, leading to OP asking his question. I cant image a situation where it would affect any program. The internal resources are precompiled, the external resources are imported as they are already. I think it could be possible to go and start wildly change existing gamefiles to random cases, but thats an unlikely and bad idea regardless of anything lol.

Thanks for info everyone.
:)