It’s not always simple.
What we have here is a can full of worms. I kid you not.
What we’re talking about is whether uppercase1 letters are treated as different letters from their lowercase counterparts. Is “LEO” the same as “leo” in an email address, URL, etc.?
The answers to the four questions above are: no, mostly no, it depends, and always.
It’s enough to drive you CrAzY.
Upper or lowercase?
Capitalization rules are inconsistent. Email addresses and most web addresses ignore case. Filenames depend on the system, so stick with lowercase to stay safe. Passwords always care about case, so type them exactly. And in writing, proper capitalization matters more than you might think.
Email addresses: no
Email addresses are not case-sensitive.2
These are all the same email address:
- leo@askleo.com
- LEO@askleo.com
- leo@ASKLEO.com
- LeO@AsKlEo.Com
The convention, or normal pattern, is to use all lowercase, so usually you’d see “leo@askleo.com”. Lowercase is such a strong convention for email addresses that using uppercase will likely get you funny looks.
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URLs: mostly no
URLs — web addresses beginning with http or https3 — are mostly case-insensitive.
These are all the same website:
- https://askleo.com
- HTTPS://askleo.com
- https://ASKLEO.COM
- HtTpS://asKlEo.cOM
Once again, the convention is to use all lowercase, so “https://askleo.com” is what you typically see. Since URLs are interpreted in many places, like word processors that automatically turn them into hyperlinks, interpreting mixed-case URLs can be a little dicey. My recommendation is to always stick with all lowercase.
Except…
This is where the “mostly” comes into play. We can make no assumptions about anything that follows a “?” in a URL. For example:
- https://askleo.com?something=something
- https://askleo.com?something=SOMETHING
- https://askleo.com?SOMETHING=SOMETHING
These could all be the same thing, two different things, or three different things. We have no real way of knowing. If you’re ever in a situation where you’re typing something like this in, make sure to type the part after the “?” in the same case as you’re instructed to. (No instructions? Assume lowercase.)
Filenames: it depends
File systems — things like FAT32, NTFS, and others — determine whether the capitalization of a filename on disk matters. Historically:
- File systems that originated in Windows are case-insensitive. (FAT, FAT32, NTFS, etc)
- File systems that originated elsewhere, such as on Apple or Linux devices, are case-sensitive. (APFS, ext4, etc.)
But it gets more complicated than that, especially when uploading or downloading files.
Since so many (perhaps most) web servers are Linux-based these days, case matters. So if you’re called to upload a file, then on those systems, “filename.docx” and “Filename.docx” can result in two separate files, even though they only differ in one letter’s capitalization.
Similarly, say you have those two separate files on a web service. If you download them both into Windows, you’ll get a warning that you’re about to overwrite the first with the second. Windows sees those as the same filename. That the first letter differs in case makes no difference.
My very strong recommendation: in filenames, always use all lowercase if you can. It removes that confusion from the equation, no matter.4
For a little more confusion, sometimes third-party tools like Cryptomator can be case-sensitive even when working on Windows native file systems that are not case-sensitive.
Testing filename case sensitivity
Fortunately, it’s an easy thing to test: create filename.docx and try to copy or save FILENAME.docx to the same location.
- If it works and you end up with two files, then the file system is case-sensitive; they’re two different files.
- If it complains that you’re about to overwrite the first with the second, then the file system is not case-sensitive; the files are treated as the same filename.
- If it works and you get only a single file, presumably with the contents of the second file, then the file system is not case-sensitive; the two files are treated as the same. The filename at the destination may or may not be updated to match the name you copied in.
Passwords: always
Anyone who has had their CAPS LOCK key stuck while entering a password knows this: passwords are always case sensitive. “Password” and “password” are two different (and terrible) passwords.
Written English
Because I make my living as a writer these days, and I’m ever so slightly a pedant, I have to include this.
Written English is case sensitive. There are many rules about what should be used when, but by and large, capitalization matters. People may still be able to read what you’re saying, but capitalization, the lack thereof, or the over-use thereof, sends subtle or not-so-subtle messages about your tone, education level, and even your intelligence. I know we’d like that not to be the case, but it is what it is.
Of course, it’s context-dependent. Text messages are definitely more informal and freeform. Written documents, not so much. Whether email should be stylistically correct is controversial and something that I’m not going to dive into… other than to say that if you want your email to be taken seriously, I recommend you write it seriously.
Bonus: CamelCase
Along with uppercase and lowercase, you may have run across the term camelcase or camel case. In this style, the first letter of each word in a string without spaces is capitalized. The capital letters look like humps on a camel. For example:
CorgiPicnicPhotos
In violation of my “stick with lowercase for filenames” suggestion, this is a common technique to make multi-word folders and filenames easier to read while avoiding spaces in filenames5.
Do this
When in doubt, and with the exceptions noted above, it’s best to stick with lowercase in email addresses, URLs, filenames, and so on.
And of course, please write English properly if you can. 
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Footnotes & References
1: This term harks back to typesetting, where capital letters like A through Z were kept in the upper of two cases, and the others — a through z — came from the lower case. See: Letter case on Wikipedia.
2: At least, they’re not supposed to be. I have vague recollections of mail systems that misbehaved in this regard.
3: Technically, URLs can start with other things, like “mailto:”, “telnet:” and others, but we’re focusing on web addresses.
4: Similarly, I strongly avoid using spaces within filenames because they can cause confusion when being uploaded, downloaded, or copied.
5: See previous footnote.
One application that had case insensitive passwords, that is of personal historical interest, was Microsoft Money.
LOL. Those were the days.
Two services I use are case sensitive for email addresses (e.g. account names.) They don’t care how you capitalize it when you sign in but every log in has to match the original.
The ASCII hex code for the character A is 0041. For the character a, it’s 0061. Those are two completely different characters. In order for programs to treat them as the same letter, they need an arithmetic conversion routine.
A bit off-topic, but on forums, posting in ALL CAPS is usually regarded as SHOUTING!
Not really off topic. It’s a case when we shouldn’t capitalize 😉
I’M A SENIOR AND MANY OF MY CONTACTS ARE SENIORS; SO I TYPE UPPER CASE TO HELP DISCERN THE TEXT.
This is the wrong way to make things more readable. Even I read your comment as being shouted at me, which is really difficult not to get angry about in return. Please see: WHY IS EVERYONE ON THE INTERNET SO GRUMPY?
Agree, please AVOID this, use the magnifier tool to read small text (win key and +) and (win key and esc) for back. Put a short key in the taskbar.
I’m a senior who prefers all capital letters so I don’t have to worry about pressing ‘caps lock’ or ‘shift’ key’. Some fonts only define capital letters and this annoys ‘ee cummings’ followers. Poor vision folks prefer function over academic form to read without a physical or software magnifier. Leo is right but since I try to write for my reader, I’m right too. LEO
It has often been shown that in the scan of normal reading our perception is more on the shape of the whole word and less on the letters that make it up. So in using all UPPER CASE one comes across both as shouting and also as sabotaging the reader’s path to ready comprehension of your message.
Having spent 15 years writing and publishing nonfiction books, I can definitely state that all caps is very difficult to read, especially for those of us on the far side of 60.
What continues to amaze me is how often the authors of contracts insist on misusing capitalization, often capitalizing “the consultant,” “the city,” and “the contract.” And let us not forget the misuse of capitals on social media, especially from the guy in the White House who stays up all night to pontificate, further revealing his lack of knowledge.
I suspect that all the examples you cite are a mistaken attempt at placing emphasis on the capitalized words, an in the case of the White House, perhaps it does further expose the author’s ignorance, if that’s possible. (The preceding is my observation, not an attempt at degradation)
Ernie
But ya gotta admit, that dude IS SHOUTING 😉
As somebody will note, ALL CAPS doesn’t improve readability; on the contrary, it decreases readability.
Having taught adult literacy for many years, one of the very first things we’re taught is that people read shapes, not letters in a word. For competent readers, the brain scans ahead and contextually ‘knows what’s coming next’ (a bit like Large Language Models such as ChatGPT, etc.).
In fact, ALL CAPS makes it harder to read as you have to stop on each word and ‘read’ it. The only thing all caps does help is the Typing Speed if the Writer doesn’t understand The Rules of english (see what I did there :-)).
To really help competent readers (and save humanity), learning the rules of English is so important and not that hard. Proper nouns, punctuation and spelling are really all you need. The rest is limited only by your thoughts and language abilities.
And when teaching others, demonstrating your skills will assist them.
If I receive something in ALL CAPS, it tells me you’re either angry or worked up, too lazy to use a shift key, in a hurry, unable to apply the rules of English or assume I have diminished abilities (for the latter, I use glasses.
fINNALY, IF I USE CAPS LOCK BUT THEN Accidentally unlock it without noticing, if i have any pride 9oh oops0, i EITHER HAVE TO GO BACK AND START AGAIN OR JUST KEEP GOING.
I’d better finish now – my brain and fingers are hurting.
Well done! Your final, and presumably most difficult paragraph gave me a very hearty laugh, while making your point excellently!
Ernie
Try german, they capitalize all Nouns, but not nationalities. My shift key is my most used key when I type in german, not an exaggeration. (miscapitalization is intentional 😉 )
Part of us reading is the shape of the words. Lower case letters have variations in height and descenders. That makes it easier to read than UPPER CASE that have no variations.
Yes. We lean to read using sight-reading and phonetics. Once we learn to read fluently, all reading is sight reading. We only use phonetics for words we don’t know.
We don’t learn to sight-read ALL-CAPS.
There are simple settings to eliminate the Shift Lock key, or convert it to function as a plain Shift, or as any other key. Search of them in your operating system.
I did this years ago. So there is no chance I will ever accidentally type something in ALL CAPS, and the times I want to do all caps are few and far between.
Many years ago when Java was a new thing, a friend wrote an application using Java on Windows.
He was (ahem) casual about consistency in using upper and lowercase both with variable and file names but Windows didn’t care so his code worked.
When he tried to run on UNIX – with case sensitive file names, his code was completely broken. So, instead blaming his sloppy coding practices, he got mad at Microsoft!
This was not as bad as having code written in C on UNIX which could distinguish both variables (in C) and file names based on case – thus expecting to foo.txt and FOO.txt to be distinct files. When ported to Windows this broke things – so they blamed it on Microsoft! Of course!
I think it’s established computing orthodoxy that all bad things that happen are blamed on Microsoft . No one could possibly get mad at the authors of C, or the developers of Unix.
… Or the coding habits of the program writer??
Ernie
Yes, probably 99.9999999999999999999% of problems are caused by the coding habits of the program writer. OK. It’s a slight exaggeration. 😉
That can’t happen in COBOL, because the Standard requires that names be case-insensitive.
But then, COBOL was around a decade or two before Microsoft existed (even Bill Gates was just a toddler), so Microsoft couldn’t mess it up.
Ah COBOL! The brilliant language that uses
add a to b giving c
instead of
c=a+b
That was designed to make it more intuitive.
The first project I worked on at Microsoft was … Microsoft Cobol.
I hope you gave people the option to use c=a+b to mean the same as add a to b giving c.
As great as Grace Hopper was, she made a serious design decision mistake with that one.
Hi Leo, good explainer that destroyed my misconception of 30 years that Win95 introduced case sensitive file names. Clearly it was just allowing long file names & spaces in the file names…
I do use spaces in file names – for clarity, but it does add complications when referring to them in a command.
The other place where spaces in file names make a mess is when downloading them from a website / service (eg. SharePoint) where that aesthetic crowd pleaser “&20” is substituted.
I’ve recently moved onto a MacBook while still running Win10 on my 11yo laptop. The machines are linked by OneDrive where I store anything of importance. Your article has me wondering what issues I might run into as a result of the different case sensitivity of file names on the 2 systems.
That’s why I prefer CamelCase. It’s so much prettier 😉
Spaces in file names work, but they can get confusing when the name is sent over the Web.
Being very old school, I use the underscore character _ in filenames. For example
2026_07_Statements
Holiday_2026_Photos
Also being a long-timer, I replace spaces with underscores or adopt camel-case, both on Windows, and Garuda Linux for file and directory names/paths. Doing so effectively eliminates any file or directory_naming conflicts with data shared between the two OSes, as well as on the Internet.
Further, when writing to inform or share with others, I may CAPITALIZE a single word to emphasize it/indicate its importance (e.g.: ALWAYS indicates the importance of doing something without fail, such as ALWAYS confirming the validity of a hyperlink BEFORE Clicking!)
Ernie
I’m a consistent user of CamelCase. It’s an aesthetic decision. I never liked the underscore method of separating words. CamelCase looks neater in my unhumble opinion. I’ve been using CamelCase ever since MS-DOS.
When I taught a course on Windows in a business college, the Linux teacher told me I should teach the students to always use lowercase for file names so that they won’t come to bite them if they open the files in Linux. That’s a good rule of thumb for those who use both Windows and Linux. Consistently using CamelCase or uppercase would also work. Consistency is what matters.
He also said to stick with 8.3 filenames and not use spaces in file names. That was 25 years ago, when it mattered.
I would respectively point out that there is space between the humps on a camel 😉
Dude, the space in camel case is space above the lowercase letters between the capitalized humps.