For some basic info about Gmail usernames generally, such as allowed characters, scroll to the end of this post. [Added 5 April 2005 - as people seem to be searching for more general info on Gmail usernames.] On using the Gmail "aliases" feature so you can effectively have more than one email address or Gmail alias with the same username and Gmail account, see this post. [Added 28 April 2005]
Tip: when signing up for Gmail, don't use any dots (periods) in the username you select.
Why should you not include any dot in your Gmail username? Because Gmail, for now at least, seems to treat your username as including all "dotted" versions of that name, however many dots there are within it, and it also seems to deliver email by ignoring any dots in the username. (I think that's an interesting bit of trivia, but then I'm sad like that...)
What I mean is, if you sign up with the username "abcd", anyone who later tries to register the username "a.b.cd" won't be allowed to - all variations of that username, whether there is one dot or more dots in between, will be considered "taken" by you. (So even "a.bc.d", for instance, can't be used by anyone else). (Yes, I know usernames on Gmail have to be at least 6 characters long, that's why I used that example - so that I won't inadvertently mention someone's real Gmail address!). Maybe Google do it this way to avoid possible confusion that could arise from different people having the same username except for where the dots are within the name.
A neat thing is, if someone sends you email, they can use your username with extra dots anywhere (or nowhere) and you'll still get the email - the person with username "abcd" would get email sent to "abc.d@Gmail.com", for example. (This also works even if the original username had dots in it).
The only point to note is that when you sign in, you have to use the version of the username that you originally registered. So if the username you originally picked was "a.b.c.d" you have to login with that exact username, dots and all, whereas if you pick "abcd" you can avoid having to type those three extra dots, and anyone will still be able to email you using "a.b.c.d@Gmail.com" if they want to.
So, I think choosing a username without dots is the best way to go. But then, I'm also lazy!
[Added 5 April 2005:] When choosing a Gmail username, bear in mind (this may be obvious to some, but not to all):
1. Minimum 6 characters (not including any dots/periods, but see below)..
2. As you'd expect, for username "user", the Gmail address would be "firstname.lastname@example.org". You can't pick one name for your username and another for your Gmail address.
2. Username can be any combination of letters and numbers. Dots are allowed (but don't bother - see above); hyphens, underscores, spaces, other punctuation, symbols etc aren't.
3. Username is not case-sensitive - capitals or lowercase or mixed, it doesn't matter (whether logging in, or as used in the email address itself).
(The password however is case-sensitive, and has to be minimum 6 characters too.)
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