What makes an Xbox gamer tag invalid?

I am trying to change my Xbox gamertag and it keeps telling me that this is invalid. How could that be? I'm sure all the characters are able to be used.What makes an Xbox gamer tag invalid?Either it is offensive or the name has been taken already, a lot of times people will create random stupid accounts and then never use them and that may take away a name you wanted to use, try coming up with something better maybe use numbers or something