Background information

Obsolete type C Bakelite wall socket (1950s)
Germany has standardized on type F sockets and plugs. Type C and type E plugs can also be used thanks to their compatibility with type F sockets.
Plug F is commonly called “Schuko plug”, which is the acronym of “Schutzkontakt”, a German word meaning “protection contact” or “safety contact”. The plug was designed by Albert Büttner, a German manufacturer of electrical accessories. It was further perfected by Siemens engineer Wilhelm Klement and a patent was filed in 1929. The decision in 1930 by RWE, one of Germany’s largest electricity suppliers, to require that all of their customers have type F sockets installed enormously boosted the spread of the Siemens plug and socket system.
Typically, type C plug sockets are not allowed to be installed in Germany: these outlets are not earthed and are therefore considered dangerous. Only type F power points are permitted because they are grounded and therefore significantly safer.
The only (and rare) exception to this rule is that an old type C outlet should be replaced by a new one. After all, if you hook up a grounded wall outlet (i.e. type F) on a two-wire circuit, the socket will not be grounded, but people will get the wrong impression that it is actually grounded. No need to say, this would be a potentially dangerous situation.
Nowadays, however, type C receptacles are not frequently installed anymore, since older properties are almost always completely rewired when they are renovated or significantly altered.