Doppelkopf: A Complete Introduction

About Doppelkopf, from BoardGameGeek:

Somewhat complex card game with variable partnerships and several different solo game types. Extremely popular in Germany. Basic mechanic being collecting tricks where different cards score different amount of points. The rule-set of gameplay differs in many regions of Germany. For example, some regions play with nines and some without, also many special points and solo games are variated in different regions.