My name is Zoë and is supposed to be spelled with a diaeresis. However, there are several barriers to making this happen on computers. Here's some of the issues and what can be done about them:

• The common ASCII set doesn't have the right symbol in it. On a typewriter, you could type an e and then backspace over it and overstrike with a " to get roughly the right effect. Some printer drivers and other software dose this both to synthesise missing characters and to create bold text. If the character set simply doesn't have the ë in it, then it's probably OK to just use e instead.
• There are many extended character sets in use that add symbols used in various parts of the world. The ë symbol is generally considered to belong to the Western European region, and is found in IBM code page 850, Windows code page 1252, ISO 8859-1 and Unicode. Quite which of these is in use at any one time is a matter of complex negotiation between the different computer systems that create, transmit, process and display the text, so a character-set and/or encoding mismatch can arise. This leads to ë being shown as ë, for example (I get mail addressed to Zoë sometimes...)
• In some systems, it's possible to construct the ë character from e and a combining character. For example, in LaTeX one can write \"e. In Unicode, there is a combining diaeresis character (U+0308) that is used after the e.
• Once there's a representation for ë and all the character sets and encodings line up, there's the question of how to actually type the character into the computer - the typical English-language keyboard doesn't have a key for ë! Here's the many different ways to get the right character to appear:
  • In languages like HTML that accept SGML character entity references, use the sequence ë.
  • In Microsoft Windows, type Alt+0235.
  • In Microsoft Windows, run charmap by finding it in the Start menu or by selecting Run and typing it in (Run can be accessed directly with Windows+r). Find the ë character and copy it to the clipboard, then paste it where you want it to appear.
  • In Microsoft Office Word, type Ctrl+:, then e.
  • In vim, in insert mode, type Ctrl+k then : and then e.
  • In the X Window System implemented by X.Org, enable compose with a keyboard settings application (typically found in a control panel) or by running a command like setxkbmap -option compose:ralt, then use the right Alt (Alt Gr) followed by " and then e.
  • In OSX, type Option (Alt)+u and then e. For further reading, here's a pretty good explanation of OSX multi-language input.

All in all, it's sometimes not worth the effort to put the character in correctly. Emails and Usenet posts don't allow for such things in the headers without some devious shenanigans that aren't guaranteed to be widely supported; IRC nicks aren't generally allowed these characters either, so in these cases I don't bother. A lot of web forms don't accept any extended characters, and I don't usually kick up a fuss. The rest of the time, I usually give it a go just to see if it works out.