If you have an extended full-size keyboard (the one with the keypad) and you’re on windows, probably the best way to obtain a special character is by using the alt code assigned to that character. Using the diameter symbol alt code on an extended keyboard It’s up to you to choose which method to use and which character, but at least for “informal” and not technical writing I think the Danish character is an easier way to avoid problems down the road. In my opinion, in many cases the Danish letter can be easier to obtain, is supported by more fonts, and it doesn’t make a lot of difference in the final result. Sometimes we’ll discuss the “real” diameter symbol, sometimes we’ll use the Danish letter. I’ll show you more than one way to do it, on various keyboard sizes and under various operating systems. Now that we know where this character comes from, let’s get back to our need for a way to type it on a standard US/International keyboard, where we apparently can’t find it anywhere.

In the Danish language, this character comes from the ligature of the diphthong oe, where the horizontal line of the e character blends into the o, generating a single character. Here's a nice page that covers the subtleties in the filename encoding of OS X's / HFS+.Windows Virtual Keyboard With Dansk Layout In fact OS X uses the spec of Unicode NFD from either Unicode version 2.1 or 3.2, depending on the version of OS X. This means that when a Unicode character such as 'é' is used in a filename it will always be normalized by the system into a regular ASCII 'e' followed by a Unicode combining acute accent, and will always take two codepoints. OS X uses Unicode NFD (Normalization Form Canonical Decomposition). Codepoints are encoded using between one and five bytes.

This means you could have two filenames that look identical with one using a precomposed 'é' consisting of a single codepoint and the other using a regular ASCII 'e' followed by a Unicode combining acute accent, thus two codepoints.

Windows does not normalize filenames using any Unicode Normalization Form. Codepoints outside the BMP (Basic Multilingual Plane) are encoding in a 'surrogate pair' that takes four bytes. Most codepoints are encoded in two bytes.

Windows vs mac unicode