While there is some appearance of symmetry in the streets generally, it does not go very far; there is hardly a right angle, or any close approach to a right angle, at any street corner.
It is generally held, as Mau has argued, that the whole town was laid out at once, perhaps during the Etruscan period, on one plan of streets crossing at right angles.

Two principal streets, those now styled the Strada di Mercurio and the Strada di Nola, are considered to be the main streets of this earliest town-plan, and to give it its general direction.

A third main street, the Strada Stabiana, which cuts obliquely across from the Vesuvian to the Stabian Gate and mars the supposed symmetry of this town-plan, is ascribed to the influence of a small natural depression along which it runs, while a small area east of the Forum, which also breaks loose from the general scheme, is thought to have been laid out abnormally in order to remedy the effect of this obliquity.[48] This theory is open to objections.

In the first place the streets (even apart from those just east of the Forum) do not really form one symmetrical plan.

Region VI fits very ill with Regions I and III.