  A simple graph $G=(V,E)$ is a $(2,1)$-circuit if $|E|=2|V|$ and $|E(H)|\leq
2|V(H)|-1$ for every proper subgraph $H$ of $G$. Motivated, in part, by ongoing
work to understand unique realisations of graphs on surfaces, we derive a
constructive characterisation of $(2,1)$-circuits. The characterisation uses
the well known 1-extension and $X$-replacement operations as well as several
summation moves to glue together $(2,1)$-circuits over small cutsets.
