The German alphabet consists of the same 26 letters as the modern Latin alphabet, plus three umlauts and one ligature:
Also, the ess-tsett (ß), a ligature of two different former versions of the "s" is used. It exists only in a lower case version.
When it is not possible to use the umlauts, e. g. when using a restricted character set, the umlauts "Ä", "Ö", "Ü", "ä", "ö" and "ü" can be paraphrased as "Ae", "Oe", "Ue", "ae", "oe" and "ue", respectively. The "ß" can be paraphrased as "ss" or "sz", although the latter is rarely used, despite the fact that it could be better distinguished from the regular "ss" letter combination. Nevertheless, any paraphrase should be avoided when possible, especially with names.
For details please see the German version of this page.