For each composable triple
\[ A\mathbin {\overset {R}{\mathrel {\rightarrow \kern -9.5pt\mathrlap {|}\kern 6pt}}}B\mathbin {\overset {S}{\mathrel {\rightarrow \kern -9.5pt\mathrlap {|}\kern 6pt}}}C\mathbin {\overset {T}{\mathrel {\rightarrow \kern -9.5pt\mathrlap {|}\kern 6pt}}}D \]
of horizontal morphisms of $\mathsf{Rel}^{\mathsf{dbl}}$, the component
