In order to define the single rule the tag is used.

Empires are widespread states or communities under a single rule.

He has violated every single rule of the old order, and he's still succeeding.

Now I see that the only way to bring peace to this world is for all of it to be under a single rule.

But here too, a single rule cannot completely hold its ground without multiple eccentricities.

He said the cases described in the medical journal broke "every single rule of safety."

The single rule at these events is "no talking."

In this way, a single rule is actually a family of alternatives.

Once again, however, there is no single rule that works for everyone.

He was breaking the single most critical rule in the world of scuba-diving.