In the paper "The display of a formal $p$-divisible group" Zink defines some objects and calls them $3n$-display. A $3n$-display over $R$ is a quadruple $P$, $Q$, $F$, $F^1$ such that $P$ is a projective $W(R)$ module, $Q$ is a submodule and $F:P\to P$, $F^1 :Q\to P$ are functions satisfying some conditions. I don't know why these things are called $3n$-display and I don't see any relation to $3$ or $n$ in the definition. Does someone know the reason behind this name?