The pronoun друг друга, called reciprocal pronoun, adds the meaning of reciprocity of an action and corresponds to English 'each other/one another'. It consists of two parts: the first indeclinable part друг and the second part друга which declines and takes the case required by the verb, adjective or preposition. If a preposition occurs, it is placed between the two parts.
The table below represents the declension of this pronoun with and without a preposition.