Let $G$ be a finite group and $N\subseteq G$. The set $R\subseteq G$ with $|R|=k$ is called a ``relative difference set of order $k-\lambda$ relative to the forbidden set $N$'' if the following properties hold: \beginlist%ordered{(a)} \item{(a)} The multiset $\{ a.b^{-1}\colon a,b\in R\}$ contains every nontrivial ($\neq 1$) element of $G-N$ exactly $\lambda$ times. \item{(b)} $\{ a.b^{-1}\colon a,b\in R\}$ does not contain any non-trivial element of $N$. \endlist Relative difference sets with $N=1$ are called (ordinary) difference sets. As a special case, difference sets with $N=1$ and $\lambda=1$ correspond to projective planes of order $k-1$. Here the blocks are the translates of $R$ and the points are the elements of $G$. In group ring notation a relative difference set satisfies $$ RR^{-1}=k+\lambda(G-N). $$ The set $D\subseteq G$ is called *partial relative difference set* with forbidden set $N$, if $$ DD^{-1}=\kappa+\sum_{g\in G-N}v_gg $$ holds for some $1\leq\kappa\leq k$ and $0\leq v_g \leq \lambda$ for all $g\in G-N$. If $D$ is a relative difference set then ,obviously, $D$ is also a partial relative difference set. Two relative difference sets $D,D'\subseteq G$ are called *strongly equivalent* if they have the same forbidden set $N\subseteq G$ and if there is $g\in G$ and an automorphism $\alpha$ of $G$ such that $D'g^{-1}=D^\alpha$. The same term is applied to partial relative difference sets. Let $D\subseteq G$ be a difference set, then the incidence structure with points $G$ and blocks $\{Dg\;|\;g\in G\}$ is called the *development* of $D$. In short: ${\rm dev} D$. Obviously, $G$ acts on ${\rm dev}D$ by multiplication from the right. If $D$ is a difference set, then $D^{-1}$ is also a difference set. And ${\rm dev} D^{-1}$ is the dual of ${\rm dev} D$. So a group admitting an operation some structure defined by a difference set does also admit an operation on the dual structure. We may therefore change the notion of equivalence and take $\phi$ to be an automorphism or an anti-automorphism. Forbidden sets are closed under inversion, so this gives a ``weak'' sort of strong equivalence. %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% %% %E ENDE %%