The History of Doppelkopf
A game played with a doubled pack, in which you often have to find out during play who your partner is.
From Schafkopf to Doppelkopf
Doppelkopf grew out of Schafkopf, a trick-taking game with a similar ranking of Queens and Jacks as permanent trumps that was played in Germany from the 18th century on. Somewhere in the 19th century, players in northern and central Germany began to play it with two packs mixed together - hence the name, which means roughly "double head".
Doubling the pack brought in what makes the game special: every card exists twice, so it matters which copy falls first, and partnerships can be secret, because nobody knows who holds the other Queen of Clubs.
Many Tables, Many Rules
For a long time every region, club and family played by its own rules, from how the Heart Tens rank to which solos are allowed. In the 20th century players' associations set down common rules for tournaments, but at home many groups still keep their own house rules.
PaperGames offers the normal game with the marriage - the core that almost every group plays the same way - and says on the rules page which additions are left out.