Before revision:  We show how a torque balance (commonly overlooked in implementations of the vertex model)  places a geometric constraint on the stress in the neighbourhood of cellular trijunctions, and requires cell edges to be orthogonal to the links of a dual network that connect neighbouring cell centres and thereby triangulate the monolayer . Torque balance also requires each internal vertex to be the orthocentre of the triangle formed by neighbouring edge centroids .
After revision:  We show how a torque balance (commonly overlooked in implementations of the vertex model) places a geometric constraint on the stress in the neighbourhood of cellular trijunctions, and requires cell edges to be orthogonal to the links of a dual network that connect neighbouring cell centres and thereby triangulate the monolayer .